@mikeh thanks for sharing this, it’s really useful and we’re fairly similar, although a little higher usage. I think that’s down to a slight issue we have that’s due to be fixed.

Follow up: had a technical survey from Aira on Friday, expecting the heat loss, technical design report on Tuesday. During the survey, we had a lot of discussions about the location of the outdoor unit. We have limited options - nothing on the back wall of the house due to a catio and french doors, nothing on the side as we have a covered lean-to, and so the front: one location will have a complex run of pipes, one close to a door and a window.  The location of the indoor unit and the buffer tank are in the "outhouse" room near the outdoor unit, where the gas combo boiler is currently located. It sounds like all radiators will be replaced, several will be vertical due to limited space. It will be interesting to see in the report what will be the replacements.
I will post with updates.

Our heat loss is 4.94 kw @ -4°
Max designed flow temperature 45°
4 bedroom semi detached 1930’s house 91.44m2
ECP A
Part triple glazed the remainder double glazed.
loft well insulated.
UPVC double glazed entrance porch.

Full electric house with two EV’s 7.4 kw solar and two 6.5kwh batteries as well as a AIRA 6kW HP. Emergency heating if we have a HP problem is Infrared panels in the lounge and our bedroom. We had these fitted to see if they would work for us, but not ideal

@andy_e you mentioned that you will need planning permission. Someone else on this forum needed the same with octopus and that’s where it all went wrong. Octopus are a great company and are looking for a simple straight forward install to keep the costs down Note that they also subcontractor their work so it’s the luck of the draw what kind of install you get. 
AIRA manufacture everything themselves and only use their own team of fitters. They also work with Heatgeek for training. The cost can be spread over 12 months interest free or on a repayment plan over 5 years. It comes with a 15 year comfort guarantee where they guarantee your design temperature over that period. If it drops below they will come out and fix it.  After sale service is fantastic. I have been on several forums and I know AIRA our watching because their area manager contacted me regards a queries I had posted. I researched for 2 years before reaching my decision on a AIRA

Our heat loss is 4.94 kw @ -4°
Max designed flow temperature 45°
4 bedroom semi detached 1930’s house 91.44m2
ECP A
Part triple glazed the remainder double glazed.
loft well insulated.
UPVC double glazed entrance porch. 
Full electric house with two EV’s 7.4 kw solar and two 6.5kwh batteries as well as a AIRA 6kW HP. Emergency heating if we have a HP problem is Infrared panels in the lounge and our bedroom. We had these fitted to see if they would work for us, but not ideal

They don’t mess around doing temporary fix’s. We may have a solenoid problem. They said if it is faulty then they would exchange the heat pump not just the solenoid. They would then disassemble the old unit to look at improving things and to make sure there’s no design problem.

@bromide 
@andy_e Note that they also subcontractor their work so it’s the luck of the draw what kind of install you get. 
 

Octopus do not subcontract their work - far from it. They have a major training set up with purpose-built facilities and train all their installers from scratch - even those who have had Heat Geek training. They are also building new training facilities in Bristol. They are very well-regarded both for their training and their installations. 
They have now partnered with Heat Geek (as has Aira) to cover areas where they have no teams operating but the majority of their work is still in house. They send their staff out of area and board them in hotels for a week if necessary. 
I'm not in the fan department but I had a good quality installation from them. 

@lucia When I first researched and engaged OE to carry out a survey some two years ago, it was all new to them but, I could tell that they cared and were enthusiastic; they were very keen to carry out a top class installation and had they been able to carry it out for me, I would have gone ahead. I have the same Daikin heat pump and siting they had recommended but the DHW arrangements are very difference due to space restrictions. Regards, Toodles.

Thanks for your reply. I love Octopus Energy and are a forward thinking company. I’ve been watching how they are building up their trading facilities and training more installers. However a couple of people have spoken to their installers who said that they were subcontractors for octopus. Maybe this was highlighted as it wasn’t the best of installs (fixed by Octopus) But both Octopus and Aira are forward thinking companies and want to get things right

My Aira heat pump installation has been well documented in this forum but I've finally got round to editing the video's I did each day. Feel free to take a look:

My journey goes on - still planning on going with Octopus - however concessions were made following the second design visit to maintain 1m from the boundary.  With the recent change in gov policy I have decided to get them back to place the unit closer to the fence and away from the door… that’s pushed things out again and the visits not h til the 19th… having no boiler makes me the worlds least popular husband /dad… let’s hope it’s worth it when it gets here!!!

@mikeh I found your series to be really helpful. We have also decided to go with Aira. We have an install date for 6-January. Let's hope that it is not a very cold spell. 
I got a copy of their detailed heat loss calculations. Which was interesting to read. We have 6 out of 8 radiators being replaced, the two not being changed are in the two main bedrooms. We have three being replaced by larger but vertical radiators due to a lack of space. I expect, as you had, that most radiators will be the same width, so that the current 15mm copper piping will be good enough. The only bit of plastic piping is the towel rail which may get replaced with some copper. Though there are just the two of us, and we only have a shower, I am glad to have the 150l indoor unit and 40l buffer/volumiser tank to go with the 6kW Aira outdoor unit.