I have been reading alot on teh forums and feel that a car should be well run in so average 10,000 miles per year. That would equate to 40,000 miles on that car. It just seems really low and would be quite wary. With a car like the GTI I would like to know it was being used and not stuck in a garage or being used by an ol' fogie tearing up the clutch.
If you are wary of low milage there may be other tell tale signs that convey the real amount of use the car has had:
gear knob, steering wheel, seats all showing signs of wear. The pedals, especially the gas pedal as this has little rubber ridges on it. If it's done low milage and all these components are still looking like they've hardly been touched then it's probably genuine. If they don't look so new then it's iffy and the owner will need to explain why.
If there's a full service history it should also log what the milage was at every service or alternatively the milage should be logged on MOT certificates.