That is very helpful, thank you both (and also MG for his telephoned response).  On balance I have decided to come up with a value for the rebuilt and certified Airey (only slightly less than a traditional build) but then deduct 5% to allow for next door not being done under the scheme.  Only an opinion of course, but at least based on evidence and research.

Happy for the thread to remain open should any others have comments in the future, or wish to refer to it.