Home assistant, pulling the same data as the app pulls (from the same source) is sufficient.

There has been quite a bit of discussion (mostly on OEM forums I think) that the flow/return temp sensors used by Vaillant on the Arotherm units are not to be trusted to give accurate readings, as they are not well calibrated. This can lead to significant errors in the COP figures reported on the Vaillant app. I opted for an independent heat meter with the full OEM kit - which should give more accuracy, and a lot more detailed information.

My HG Z-D price was a fair bit higher than quoted by @l8again above, but my primary pipe run is quite long and complex, which probably accounts for this.

The only piece of the HG offer that is missing from the survey is any mention of heating controls? Am I missing something?

No you arent, because you dont need any, and furthermore shouldn't have any other than the controller that comes with the heat pump.  Third party heating controls are, with a very few exceptions, a way to cripple your heat pump performance and often reduce comfort levels. 

The Vaillant controller is very flexible and anything you add will detract from, rather than augment, performance.  Some monitoring (beyond that which comes with the app) is useful for diagnosis if you are that way inclined.   Home assistant, pulling the same data as the app pulls (from the same source) is sufficient.

I assume greater water volume must count for something in system design and bigger radiators should result in a lower flow temperature?

Greater system volume helps with cycling, by increasing the length of the cycles.  Bigger radiators = lower flow temperature = lower running costs.  £1150 for 6 rad changes is good value, but illustrates how simple this upgrade is provided the rads fit in the space available!  Im not a plumber, well past my most agile years, and even I can do 2 in a very relaxed day.

The cost came in at £2250 with no radiator changes and a 5kW Valliant HP with a 5.86kW heat loss. The installer favoured a 7kW heat pump to reduce de-icing cycles. We then looked at options. I asked whether it would be possible to upgrade a K22 radiator panel in our family room. This was priced as a customer option and we agreed to use the family room radiator in our dining room. The installer then ran the survey again. 

This came in with a revised cost of £3400 with 5 further radiator changes; a Valliant 7kW HP and a minimum SCOP of 4. The ‘delta’ cost of £1150 for effectively 6 radiator changes doesn’t seem like bad value. I assume greater water volume must count for something in system design and bigger radiators should result in a lower flow temperature?

The only piece of the HG offer that is missing from the survey is any mention of heating controls? Am I missing something?

On our coldest day so far (6th Jan) it was looking like this:

Flow temp was about 50. Average heat output 5.5kW, and the house stable at 19 C and about -5 outside.

The work required:

Removing boiler. Capping off the old boiler flow/return in our downstairs loo. removing gas pipe (and meter).

Heating header tank removed from attic, and replaced with new expansion vessel and PRV (in attic). PRV piped to outside though soffit. Cold water tank was retained - DHW is a vented system.

New diverter valve, OEM flowmeter, filling loop in airing cupboard (plus Vaillant control panel etc).

28mm primaries run from airing cupboard, up through the attic across two bedrooms and down the outside of the house, then over a side passage to the heat pump.

Outside, a couple of paving slabs were removed, soak-away dug, filled with a plastic cage, and capped in concrete to form a base for the outdoor unit.

New electric supply installed from understairs cupboard (new small CU including RCD and MCB, teed off a henley block), and run around outside of house.

So actually quite a lot of work was involved - but mostly outside and in one room inside.

So not really 'Zero Disrupt' - more a 'Less Disrupt' in reallty.

We subsequently had our bedroom radiator replaced when I realised that the TRV was not functioning allowing the room to overheat. We have our bedroom set to 18, and less overnight, and so is cooler than elsewhere, limited by a TRV. Our bedroom was overheating and I realised that the TRV was not closing properly and needed replacing. Since that radiator came out as rather undersized on our heat-loss survey, we got out installers back to replace the whole radiator (upgrading a 11 to a 22) rather than just the valve.

How zero disrupt was it? Did you need upgrade of the pie work directly from the ashp, to get the full 8kW from the unit? We needed a length of 22mm replaced by 28mm, because of the way the house was plumbed. Although only a third of the house length it was convoluted and a lot of work for the guys, (and new bathroom vinyl for us).

Do you need the full 8+kW?, we do need it to run at maximum eg in the cold January spell.

That seems to be the way OEM has set up processing previews of links... but the link does work and takes you to your page. Here's the direct link for others.