So, by means of a brief update, I've made some progress over the past week or so and the picture is becoming a little clearer.

Octopus have provided their calculations, floor plan and radiator schedule, which has given me a base of knowledge to work from.  I've also had productive conversations with EDF (who use an acquired company called CB Heating for their Heat Pump work, and who've come across as very knowledgeable so far), and had an initial quote conversation with E.ON.  In addition, I've spoken to several locally based independent installers, one of whom was able to carry out a house visit at short notice while I was at home yesterday (not a full survey, but it included an examination of the various planned installation locations, and a review of the pipework in various areas of the house).  I have two similar visits booked for next week.

It's becoming increasingly clear that the installation itself is a relatively straightforward proposition for which there are no overall concerns.  The common challenge for everyone is how to deal with the conservatory within the design, and this is leading to a range of options/solutions, so far.

The original Octopus survey treats the conservatory as a habitable space, with a 22 degree temperature requirement, based on the option I chose between doing that and treating it as non-habitable with a 16 degree requirement.  This is essentially why their 11kW Daikin solution potentially looks oversized.  As I've had more discussions with other installers, it's becoming clearer that not targeting normal room temperature for it might be the more logical option.  The EDF consultant probably summarised it in the simplest way, in effect, "if we include the conservatory as a habitable room, the remainder of the house will be heated less efficiently, as the system has to work harder for the conservatory heat loss.  The alternative option is to design a system for the rest of the house and accept that the conservatory will never get up to target temperature, but the design will be better suited for the main part of the house."

The independent installers are saying similar things.  For example, one has suggested looking at an electric air heater for the conservatory, when required, and I'm expecting to have a conversation with EDF's senior designer this week to discuss options (an unusual step in their normal quotation process, but a good indication that they're looking at our specific situation).  They've mentioned one option of perhaps using TRV's set low in the conservatory for the periods of non-use so that it effectively gets bypassed when not required, so that it is treated as an occasional room, which is more like how it gets used in reality.  From a heating perspective, we spend more of the year trying to keep it cool, rather than warm.

Logically, at some point down the line, the logical plan for the conservatory would be to replace the roof panels with insulated ones, at which point it becomes more like a 'normal' room in terms of temperature fluctuations, so the system design we choose needs to also consider this likelihood over the life of the heat pump.

If nothing else, getting the Octopus survey done at an early stage has at least enabled a better quality of initial conversation with the other providers, and is starting to result in a bit of variety in the solutions suggested.  There is undoubtedly going to be an element of compromise in whichever solution we go with, which also needs to take into account how we are likely to use the conservatory 'in real life', which puts it somewhere in between the standard options of habitable and non-habitable spaces.

In terms of practical next steps, I'll likely get a 2nd full survey done by E.ON, as this requires a refundable deposit of only £50, and by the end of next week I should also have had 3 60-90minute site visits from the independent installers, which should result in a reasonable range of quotes and calculations to make a better assessment from.  Boxt still remain the cheapest overall quote, although the gap has narrowed considerably to the next option, as more quotes are being added to the mix.  However, their requirement for full upfront payment to commence any form of site review is likely to rule them out of consideration, as there appears to be no sensible way to have the necessary conversations about tacking the conservatory without doing so.

One addition point worth noting is that I've been very upfront with every company I've spoken to about the steps I'm taking and the fact that I'm speaking to other companies.  The sales consultants at both Octopus and EDF, who've been the contacts for the initial quote generation, have both been very accommodating in terms of time spent on the phone and both have made comments to the effect that if I end up getting a heat pump, regardless of who I use, then they can take satisfaction from that.  It's only a 'failure' for them if I end up choosing a gas boiler after having researched heat pumps.  I have to admit, I found this attitude very refreshing to hear.

That all sounds very logical.

The only question I would ask is how well insulated is the conservatory from the rest of the house.  If its not well insulated then you wont succeed in maintaining much of a temperature difference and not heating it may cost more money than heating it.

It was added by the previous owners, but the conservatory is built on to what would have been the original back door for the property so has the original external wall separating it from the kitchen, accessed by a single door, and with two, previously external windows to either side of this.  In use, we can keep the doors and windows closed and it effectively feels like a different temperature zone in there in cold weather (it has a separate thermostat in there, so is heated separately to the remainder of the house currently, in terms of thermostat control).

In summer, when it operates more like a greenhouse, we can equalise the temperature to some extent by leaving these internal doors and windows open.  The majority of the surface area wall which links it to the house will be the original cavity insulated brickwork, so the insulation between the two is adequate, from the perspective of keeping them as separate spaces.

Posted by: @sheriff-fatman

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It was added by the previous owners, but the conservatory is built on to what would have been the original back door for the property so has the original external wall separating it from the kitchen, accessed by a single door, and with two, previously external windows to either side of this.  In use, we can keep the doors and windows closed and it effectively feels like a different temperature zone in there in cold weather (it has a separate thermostat in there, so is heated separately to the remainder of the house currently, in terms of thermostat control).

In summer, when it operates more like a greenhouse, we can equalise the temperature to some extent by leaving these internal doors and windows open.  The majority of the surface area wall which links it to the house will be the original cavity insulated brickwork, so the insulation between the two is adequate, from the perspective of keeping them as separate spaces.

In that case treating it as a separate space seems eminently sensible to me!

I took a look back through the thread and I posted earlier sone sense checks with gas consumption you might like to do.  Im assuming you will now be looking at 5-7kW not 11kW.  Its best to avoid an oversized heat pump if you can.  That said if it does end up oversized you can heat the conservatory!