If the work needed Building Control approval, has that been signed off? This only covers key parts of the construction and structure but at least should put your mind at rest that the structure is built to minimum standards of safety, strength and attributes such as damp-avoidance and insulation.
If you are still in contact with Building Control you could try and get them back for another visit - afterall fees will have bee paid for this service.
If not you could try these people.
http://www.labc.uk.com/site/scripts/documents. php?categoryID=287
The explanation about LABC says it all - it is the private business arm of the Building Control inspectors. It is now possible to employ LABC to undertake building control services in place of using your local authority inspectors so I'm surmising they may also be able to undertake independent assessments, without the add-ons of a 'professional chartered surveyor. Worth a call.
However I would question what you are thinking you will do with the information. If you owe him nothing, taking him to the small-claims court for redress is probably the only option - and they getting money out of him is yet another issue.
Unfortunately you might have to put it down to experience.
What's wrong with the structure?