Your display is confusing to see on a Grant controller. The symbols you have highlighted ('On' means a Timer program is currently running; the clock symbol indicates hot water schedule is active) indicate that the Controller itself is being used to program Central Heating and Hot Water times, but Grant do not suggest this in their installations. All control is performed by third-party devices, external to the Controller. Essentially, the controller you show is used for information only, and as a simple on/off device.

To understand how to program that controller look in this manual (page 26 onwards): https://www.manualslib.com/manual/1291026/Chofu-Aeyc-1639u-Ch.html?page=26#manual

However, as I say, Grant do not install their HPs with that Controller in use to control the system. The HP should be installed with those functions disabled.

@mikefl Thank you very kindly, this is extremely useful. Unfortunately I "Inherited" the system and I don't know how it was originally setup but I am beginning to suspect it had an external controller like a nest hub who just commanded the heatpump to turn on and off.

My issue is that this controller is now gone (if there ever was one), and all I have is the chofu one which was installed inside the boiler room, which fuels my belief this is not how the system was supposed to operate as it would not make any sense to get the temperature reading from there.

My problem is that when I press button 2 and On is displayed, it looks like the system overshoots and keeps running and heating past my set temperature and when the time slot should have kept it off. What could cause this? Can I check or adjust some parameters to see if it functions correctly and troubleshoot this behaviour?

Or should I just ignore the technician I contacted and was against installing a nest type external controller as it was not suitable for heatpumps? Is the installation of that an involved process?

It's hard to tell. Having the Chofu controller in the airing boiler room is generally where they are placed - and as such, the "internal temperature" they indicate is pretty useless. And that's how Grant want it to be. If an external Hive (or similar) was in place, I would expect there to be some indication elsewhere of its presence. 

If installed by Grant, it's not possible to set programmed times into the controller - so far as I understand it, you'll just hear a beep from the controller if you try any of the disabled functions. It is possible to re-enable all of the Chofu functionality, but this involves some re-wiring, and is not a simple task. The features are all controlled from the parameters programmed into it; but simply changing these parameters wouldn't reverse the wiring situation.

If the HP was installed to Grant's requirements you should have a CH programmer, and a DHW timer, both 'external' to the Grant controller. These will be wired to the EP001*, and the output from this will go to a diverter valve (to determine DHW or CH) and the central heating pump. You should also have a low loss header. If these aren't present then someone at some point has amended the system to revert it to its original manufacturer's configuration. But you need determine the actual configuration: Grant, Chofu, or something else.

Can you post any pictures of the internal setup within the boiler room?

*The heart of Grant's re-working of the controls is its EP001 Wiring Centre and it's this wiring which sidelines the Chofu controller, electrically ensuring that a DHW demand denies any CH demand, and enabling external CH programmers to be used.

@mikefl Thank you again for the very detailed reply, you are an absolute legend!

In my case I am able to set a program using the controller, I have created a schedule for week days and for the weekend and set a target and setback temperature for the room heating. This so far does not appear to have any effect.

I can also create a schedule for the DHW, with a target and "setback" temperature. This seems to work alright but I am not 100% sure as the other half has gotten pretty fed up with all this and we just used the electrical plunger now to reliably heat our water tank.

I googled low loss header, I don't think we have that in the boiler room, the only similar looking thing is a manifold in the closet downstairs, I assume that is just the way the underfloor heating was plumbed.

Right as I'm typing this and looking at the photos, I noticed there is indeed a thermostat looking thing between the Gant box and underneath the immersion timer. I never paid any attention to it before, maybe this is the missing piece to the puzzle 🤔 Can't believe I'm getting this excited over a thermostat.

If you have any more advice on how could I figure out I'm all ears. Hopefully there is a quick test that will show what is happening, as my previous attempts involved making a change and going to work and when we returned we found our bedroom either turned into a coldroom or a sauna, I would like to have a shorter delay before I get a reaction from the system, or my wife 😬

Looking at your first picture, showing the HW cylinder, you can see (from the top) the HW Timer and Thermostat (these are controlling the DHW times), then there's the Grant EP001 wiring centre (yellow warning label on it), then lower down the immersion heater element (only used as backup and/or Legionella), and a 2-port valve (the box with a crescent shaped orange light on it - assume this is only lit when the CH is on).

The third-party controls would be the other end of the collection of cables coming from the EP001. I can see them tied together leading off to the right on your picture 4. I'd expect the third party CH timer and programmer to be the other end of those. At most 2 will be coming from the Heat Pump itself, but there are at least 4 cables I think. They ought to be in an easily accessible location. 

Ditto what mike has said: there would normally be a separate timer (programmer ) to set the on off times for heating and dhw, and a room thermostat (or you could have a combined thermostat and programmer for room heating).

If the previous owner has modified the settings to use the grant remote display ( the controller in your 1st pic), then it's conceivable they removed the timer or replaced with an on off switch - hence it would be good to follow the control wires and see what they're connected to.

Just for info, you appear to have a buffer tank rather than a low loss header: the squat white cylinder on the floor next to the hwc.

Cheers gents, my mission for tonight will be taking off the cover from the Grant box and attempting to trace the wires.

I wouldn't take the cover off the EP001 - just trace where those 4 white electric cables are going to. At least one will be carrying the control signals from the external unit to the internal controller, but  one of the others should be ending up at a room programmer; or an RF (or Wifi) receiver which is (or was) getting messages from a room thermostat. Maybe two controllers - one for UFH and one for rads? Not sure what you have.

Progress update, yesterday I started following the cables, this did not take long at all as I literally run up against a wall right away.

The cables go in a hole and then I presume run down as there are not other upstairs rooms behind that wall, so I spent some time downstairs looking for boxes, receivers or any sign of thermostats, unsurprisingly I found none.

With no other leads to follow I decided to open the Grant box and look inside.

https://ibb.co/8s54pyG

Well 🤬 me, I think I found the problem!

Or at the very least something that needs fixed.

2 of the wires are fake and are there just for looks and the rest is a total mess, at this point I was so pissed I threw in the towel.

Any ideas on what is going in that box? Did someone try to do the Chofu mod and botched it?

Yes, that certainly isn't wired as Grant want it to be. Zones 1 and 2 are wired together (which isn't that unusual, if somebody wants both zones to operate at the same time, but it wouldn't have been wired that way at commission); zones 3 and 4 are empty (which is fine); mains supply in the next set of connections; but the right-most doesn't look right, and the important thing is the HP connections should be wired off the top right-hand connector block, but none of those look populated. Your Controller must be connected to the HP outside of this wiring centre. Even if there had been an attempt to revert the Grant back to Chofu, the controller being in the boiler room (which I think it is?) is useless - you'll never get a proper 'room temp' reading.

Here is a the wiring for that version of EP001 so you can see (section 3.5) that yours doesn't match that. 

Looks like that's what they've done: the wires that would normally go from the top right of the ep001 to demand heat / demand hw have presumably been re-purposed to turn on the zone valves /circulation pumps at the command of the hp,  is my guess.