I know the previous owners are pretty poor and I
wonder how much compensation we could get anyway. Do we use their
solicitors statement to show the age of the conversion to our benefit
when
we sell up?

They've defrauded you. They misrepresented it and it's worth less than
you paid them for it. That doesn't justify your misrepresenting it when
you come to sell up. Assuming it's still standing.


so I guess our neighbours statements are our only chance.
And any other neighbours who've been there that long, or any previous
neighbours you can trace, and the 2nd last owners, if you can trace
them. Look in the old register of electors in the central library for
names. Ask a solicitor if it's worth the effort.

Or do I just leave it - there does not appear to be any subsidence or
sagging to the loft, plus

You need a surveyor to survey it. It may be unsafe. What else might
they misled you about?