Maybe I am being too cautious but I need to know if my surveyor did an
accurate job by not mentioning any probs with the conversion.

Did you have a full structural survey? If not then you don't have much case
against the surveyor.

Can I get indemnity insurance now, then pay a structural engineer to come
round and have a look, then claim on the insurance if it needs updating?

That would be a fraud on the insurer so no you cannot. A contract of
insurance is regarded in legal terms as "of the utmost good faith" so you
have to disclose any information to the insurer that might affect the
premium.

You might well have a case against the seller. Even if they don't have much
money they may have sufficient equity in their present house. No a
particularly nice thing to do to someone but it may be justified if they
have deliberately mislead you.

Peter Crosland