WilliamsClio forum should have the definitive answer.
My understanding was that only the Williams 1 didn't have a sunroof.
What colour is it? The changed throughout the 1,2,3 models.
Ask for a pic of the engine number and gearbox code and that should help determine what it is. Stance of the front wheels in relation to the front arches should also indicate if it's got a Williams widetrack.
What does it come up as on the MOT/Tax checker?