Yes a structural engineer/surveyor. It's possible a regular surveyor could also do it but they may just end up recommending you get a structural engineer out as well, to assess the full extent of the problem, if it turns out to be something definitely needing work doing.
Would that be a structure engineer (current), I take it?
Thanks for the pointers too all.
Although the neighbours are currently building an extension, it's at the other side of the house. Probably worth checking out though.