I believe that if your neighbour does carry out the work, potentially his property will have what's known as a 'flying freehold'. This can cause alarm bells to ring when selling.
I hope I am not hijacking this thread but it just seems to be so similar my situation it seems silly to start another - I am in a similar situation but not quite the same!!
The ceilings of my house also slope down to the eaves, the horizontal area of the loft is insulated but there is no...