Hi
If there is doubt that it is done properly, I personally would get a structural engineer.
A surveyor, IMO, isn't up to the job. An engineer is the bloke who would be able to ratify/condemn any structural part of the building, whether it was done in a standard way or not, within reason.
A surveyor may feel happy commenting if a modification was done in a standard way - but I'd go for the engineer. Try yellow pages and phone a few and see if they will quote for inspection. IFAIK, the engineer should be backed by insurance, so if he gets it wrong, you may be able to claim for remedial work - ask up front about this.
If you are selling - is it your residence? Do you know when the chimney might have been removed? Try visiting your local Building Control Office (Borough Council) and see if they have anything on file for your address.
If it was done under building control and there is a signed off certificate, that should solve your problem...
HTH
Tim
Sure someone with more experience of this will be along soon - so wait around for a bit and see if anyone else replies.