If it is relatively convenient for you to acquire them, photos of the signs would be very useful.
If, as mickR notes, the alleged contravention is not on the signage, this is an important point to emphasise - the £100 charge is for failing to comply with the terms and conditions offered in the contract, if the term you are alleged to have breached is not in the contract, this is a key point. They seem to be seeking to argue that this is an implied term, but I can't think of a good reason not to include it in the signage, as most parking companies manage to do.
An unrelated point whilst I remember: has the medical practice been approached yet, to see if they will get the charge cancelled? This is always worth a go.