I'm surprised at your experience with Aira's sales people,  I was very happy with the way they conducted themselves (as with all the other I contacted except when the contact at Octopus changed). I think Aira operate on a sales person giving you the basic picture and fixed quote for free followed by a detailed and paid for survey. Others do the full survey, they may ask for payment. Worth noting for me that Aira's full survey recommended more radiators that the sales person had estimated, but there was no price change.
Also, just because you have three year old radiators doesn't mean they are the right size for a heat pump. It will be interesting to hear what your other surveyors tell you.
Dropping the 15 year guarantee is an interesting one. Maybe Aira think that commercially they can get a bigger market share by dropping the outlay in by doing so. I used a rule of thumb of £200 p.a. service cost, so 10 years gives them £2k to play with.
As for the gas boiler cut off date target, I speculate that this gets the government off a hook to hang itself by when it isn't being reached fast enough at the next election, particularly give the cock up on the winter fuel payment announcement. I suspect that softer nudges will come into play such as decoupling gas and electricity prices. Also I read that putting gas heating in new builds is likely to be uneconomic under the FHS - Future Homes Standard.

Aira completed the technical survey for our property today. The assessor was very thorough and probably spent more than 3.5 hours going through all the minute details. Really impressed with his knowledge and attention to detail. If the installation team is as good as him, we are onto a winner. Overall, no curve balls in our case. I should get the detailed technical design soon and they can go forward with the installation as planned in a week.

A bit more progress on our side. Aira sent the detailed technical design on Friday. They are going for 46 degrees flow temperature and 38 degrees return temperature. After carefully going through the design, I had a few queries. After a bit of to and fro, they are planning six radiator changes, and reuse some of the downstairs radiators for the bedroom floor. I wanted to reduce the carbon footprint as much as possible, so rather than go for all new K22 radiators upstairs they are going to repurpose the larger K11 radiators from downstairs. The technical team has been very knowledgeable and responsive. The good experience continues and hopefully they can start the installation on Monday.

With a temperature difference on flow and return of 8C is the Aira able to operate comfortably with that? or are they using hydraulic  separation I.e. 4 port buffer aka low loss header. There are many arguments against that.
Totally agree with reuse of perfectly good radiators, we did with one but only one was the right physical dimensions.

All Aira systems are designed with a 4 port buffer. They say the heat pump has been designed from the ground up optimised to use a buffer. They have an additional temperature sensor on the buffer that the system reads. I don't think most other systems have this. My Aira system performed poorly to start with because this buffer sensor had not been installed correctly. It was reading ambient temperature not the buffer temperature and the heat pump was working hard to make the buffer heat up when it was actually already at target temp. Now the system works much better. I've put my own temperature sensors on the buffers flow and return because i'm curious and it is generally 8 degrees difference between flow and return. No problems with heating the house at -5'c a few nights ago and I've not seen Flow temp higher than 43'c. I can't tell how regular it was defrosting though. the Data in the app isn't detailed enough to tell and I'm not standing out in the cold to watch it 🤪  

@mikeh Spoilsport!😉  Actually, I have only been outside at the appropriate time to see our pump defrost once in nearly two years - but it is quite spectacular to see! Toodles.

@judith Our design includes a 40l buffer tank, and if I remember correctly, I think they mentioned a buffer is included by default. They have mentioned a SCOP of 4.1 in the design spec, so would be interesting to see what we achieve in real use.

Yeah we have a 40l buffer too. Problem is the App doesn't tell us the COP or SCOP. So we can't really tell. Hope they add this soon. They keep adding new features so finger crossed.