Recently we had a damp patch appear and get larger very quickly, so we knew it was not rising damp.

We thought it may have been a leaky pipe but a plumber confirmed this wasn't the case, so it could be penetrating from outside. The damp is exactly where an extension joins the house, so we think it could be that. However, it is difficult to tell because there is only a gap of a foot or so between our outer wall and the neighbours wall. I have tried to take some photos and it looks like there is a large gap where the extension joins the old house.

So I have a couple of questions. Who is the best person to assess whether the join between the extension and house needs fixing? Secondly, we bought the house in May and the surveyor didn't pick it up in a full home buyers report, so do I have a case for a refund? I'll try and attach some photos

Looks like an expansion joint between the two buildings, so probably not the cause.

A flexible builders mastic will fill/cover the gap.