(I noticed the comment from GrahamF on the temperature correction and pleased he inserted the parenthesis to emphasise the mathematical priorites. They are not in the manual.) 

Early days for us as the ASHP has only been running a couple of weeks. I have it connected to Home Assistant with a standard Econet300 integration but I am attempting to enhance the REST integration to obtain other metrics including the energy usage. 

I should mention that we had a GSHP in our previous property in the Brecon Beacons. This had a Raspberry Pi interface. In 16 years the technology has advanced tremendously!

There are a few aspects of this particular system which confuse me:

1. How hysteresis works - certainly not like I understood it as our DHW temp never reaches the target;
2. Why circuit 1 assumes radiators as emitters;
3. Why Econet24 reports Circuit1's actual temp as that of the ASHP - Circuit1 cannot have a temperature sensor;
4. Why Circuit2 has gone missing from Econet24(!);
5. How the heat pump's target outlet temp (Service params - read only values) is calculated;
6. Why are there temp corrections set on DHW, Circuits, "Setpoint temp. correction - heating" in the Service parameters.

If anyone can point this old IT techie at a decent heat pump glossary, I would be very grateful!

I know that the system is configured to give DHW priority, but I think this one is configured such that the target is not reached but the compressor stops. (The hot water cylinder is not a Grant model as we needed a solar coil. It is a UK Cylinders 200l model.)

The house was only built in 2010 so has decent insulation and the like. I shall do some investigating and post my "discoveries" in this forum. 

Thank you to all who contribute!

That's very helpful.

Clearly the control has various unique features. The ability to have immediate hot water seems premium. 

Useful how the Thermostatic Pump Blockade allows fine tuning of the flow temperature. 

The thing is the way Grant changed from R32 to R290 highlights a different approach to product management. I wonder if that creates some interesting management of replacement parts...

I suggest you read the Grant Aerona Smart Heat Pump System Controller Installation & Operating Instructions.  Below, I will refer to section numbers in that manual.

https://www.grantuk.com/media/6037/grant-aerona-smart-controller-installer-and-operating-instructions-uk-doc-0203-rev-20-october-2024.pdf

In answer to your points:

1. You have to use the Aerona Smart Controller with the Aerona 290.  Yes, the associated display does include a thermostat.  It is one of the more user friendly displays.
2. The system comes with an external temperature sensor.  You set the weather compensation curve as described in 7.13.
3. Thermostat control (7.14.2): when the room reaches its target temperature, the system behaves differently depending on the setting of Thermostatic Pump Blockade":
   • When Blockade is set to "On" and the temperature is reached, it turns the heat pump off.  
   • When Blockade is set to "Off" and the temperature is reached, it continues running with the water flow temperature by the amount set in "Decrease Water Temperature". See 7.8.1.1.
   • You can set day and night temperatures - see 7.3.1.3.
4. Domestic hot water:
   • You can set the hot water temperature (7.2.3) to just one value.
   • You can schedule the DHW in half hour slots (7.5).  
   • There is also a Boost function to make it run at any time.  It will continue running until it reaches the target temperature, or until you turn it off (7.3.2.2).  I would prefer a half hour timer on it as well.
   • You can schedule the weekly Legionella cycle (7.4).
   • You can configure a hot water loop, which I have.  It pumps water from the tank around a loop to near the kitchen and back into the tank.  Whenever you turn on the hot tap, you have nearly instant hot water for washing up.  This is great in theory, but the circulation mixes hot and cold water in the tank, which makes it all tepid.  You can mitigate this by scheduling the loop for meal times and setting the duration and frequency of each pumping cycle - e.g. pump for 1 minute, stop to 2 minutes, pump for 1 minute.  See 7.5.4.
5. There is no Holiday Mode button, at least in my version of the firmware - a shocking omission!  You can turn the heat pump off when you go away and restart it remotely using the phone app.
6. The Smart Controller is hardwired into the heat pump, the display and the Smart Hub (see 10) which connects to the Internet.  It runs fine when the Internet goes down, but of course then you lose remote access.
7. The User Settings/Energy Monitor displays the instantaneous electricity consumption, heat output and COP.  It also displays the SCOP, which presumably has been measured since installation or maybe since the last power up or firmware upgrade (not sure).  I  should warn you that the COP is probably overstated, as is common with heat pump manufacturers.
8. The phone/tablet app and the ECONET24 web site can display graphs of compressor frequency, flow and return temperature, outdoor temperature, room temperature, hot water temperature, 3-way valve setting...  Frustratingly, you cannot produce a graph of electricity consumption, heat output or COP.  However, if you are happy to interface it to Home Assistant, then you can produce graphs of all of those. 
9. I use a Shelly Pro EM-50 to track electricity consumption.  It agrees closely with the heat pump's own measurements.
10. The manual refers to a choice of "Installation Pack" (4.2).  Ask your installer what is in the heat pump pack, which seems to be an outdated Grant term.  You don't need flow/return temperature sensors or external pump, because the Aerona 290 includes them.

We plan to have a single home temperature driving the pump, but it would be nice to vary that temperature in terms of day and night. Nowhere near perfect and we may not vary it much anyway but will help peace of mind..

So the 'nice' may well limit you to a small subset of heat pumps unless you want everything other than the highest to be on/off control on (built in) thermostat. 

You will need to read the manuals pretty carefully to determine whether changing the set temperature changes flow temperature or just invokes the inbuilt temperature sensor/thermostat function, ie acts as on/off.  I can say for certain that Vaillant does the first, cant be certain/remember others.  I suspect Homley, Passiv, Havenwise, Adia as addons do the first also.  Not sure how much it matters in practice if setback is small, but it might matter quite a bit if you want to do a set forward because it means that most of the time (when not set forward) you are operating on thermostat.

The smart controller acts as a thermostat, but you can also connect in a wireless thermostat if needed.

Grant do only allow you to have one thermostat assigned to each heating circuit. The controller supports up to 3 separate heating circuits.

You can set a ‘day’ temperature and a ‘night’ temperature on the controller and program a schedule to change between. You can do this for each day of the week individually.

Weather compensation works with the external sensor included with the external unit, or you can connect a separate external temperature sensor to the controller I believe.

DHW schedule is fully programmable in half hour slots for 7 days of the week.

Legionella cycle is supported with the included immersion heater in the DHW tank, with a programmable day and time to run once a week.

The smart controller is completely standalone and doesn’t require Internet access to function. Internet access is only required if you wish to use the econet app to monitor or adjust settings remotely.

The smart controller shows electricity usage for the DHW and space heating separately along with the heat output generated. It lists an instant COP and average COP figure to show efficiency of the ASHP.

I would question whether you actually need multiple temperatures at different times in different rooms. Heat pump heat is much more even, and it's more efficient to keep everything at a constant temperature than have it swinging about all the time (Heat Geek has a video on how zoning is actually counterproductive). We just manually adjust the temp at night; that's something possible to adjust via Modbus so I expect the smart controller could do that automatically.

We plan to have a single home temperature driving the pump, but it would be nice to vary that temperature in terms of day and night. Nowhere near perfect and we may not vary it much anyway but will help peace of mind..

From what I can tell, on the R32 the smart controller is effectively a prettier settings and status UI than the Chofu LCD programmer.  For example, instead of going to the Chofu programmer and holding buttons to go into installer mode to change setting 3141 to 0, you would go into a touchscreen menu and choose 'Legionella day' to be 'Monday'. But what happens during the legionella cycle is still controlled by the HP, not by the smart controller - the smart controller just talks to the HP via Modbus to change the settings.  That means if the smart controller goes away for some reason, the HP carries on as normal - you just can't change the settings.

I don't know if the R290 works any differently. From a brief look at the smart controller installation instructions it seems that it forms the traditional wiring centre, with only Modbus going back to the ASHP. That implies that the decisions whether to turn on/off valves, pumps, etc are being made in the smart controller, or made in the HP and being sent to the smart controller via Modbus and the smart controller drives the relay. That means a failure of the smart controller would cause the heating to fail, but doesn't mean that internet access is required (it looks like it's an addon so I suspect it isn't).

(On the R32, I have reverse engineered some of the Modbus status but not enough to control valves, immersion from Modbus values. I'm not sure if they are being reported that way - but I haven't put a lot of effort into it)

I would question whether you actually need multiple temperatures at different times in different rooms. Heat pump heat is much more even, and it's more efficient to keep everything at a constant temperature than have it swinging about all the time (Heat Geek has a video on how zoning is actually counterproductive). We just manually adjust the temp at night; that's something possible to adjust via Modbus so I expect the smart controller could do that automatically.

Again, for hot water I just leave it on 'comfort' at 44C which means there is hot water all the time, and the HP comes on if the hot water falls below that. For this it needs a temperature sensor in the cylinder - I think Grant include that as part of the Smart Controller setup?

If you are trying to make it run at particular times to suit a variable tariff then that's another thing, but I think it's better just to leave it to get on with it.  It makes for a more comfortable house because you're never cold or have cold water.

Just hoping some users (no pressure @grahamf @davec @mikefl @allyfish @prunus ), out there can share their experience about the controls. At the moment, they seem to go by the name of Aerona Smart Controller?

I believe our requirements are common..

For heating, I believe all we will need is to have it working as a programmable thermostat with different room temperatures for different times of day and also vary by weekday (not necessarily for 7 days, but to have some flexibility). And to use weather compensation with an external sensor. 

For the hot water, probably just set one target temperature and the times of day we will need it to warm the hot water tank to it. And the legionella cycle of course. And also have a holiday mode button..

Essential is for all to be "wired in" regarding the controls are managing the pump, etc - without dependencies on the internet during operation. 

A nice to have is if the controls would also provide some information on overall consumption and cost.