The surveys generally come under three headings, 1. Valuation report - very basic info where surveyor is really just giving a valuation for the mortgage lender. 2. Homebuyer's report, normally about 3 or 4 pages pointing out faults of a more serious nature and possible future problems that are likely to occur. 3. Full structural survey - these are normally quite extensive and are worth getting on older properties but still in a lot of cases the surveyors do not for various reasons gain access to roof voids or under floor coverings to look at conditions of floorboards, joists etc.
IMO if I was buying a flat, I would probably go for a Home buyers report unless I was feeling a little suspicious of faults being recently covered up. I always go back for a second viewing before instructing a survey and take my time sometimes finding a few problems to ask the surveyor about before he has a look.