The signage was obviously not that discreet as you were aware of the restrictions, and they were exceeded. I�m not quite sure, therefore, on what basis you would argue the case.
The issue of signage has been tested in court. It has been ruled that when using a car park the driver has a responsibility to establish the restrictions before leaving his vehicle and the restrictions do not have to be in bold letters posted every two feet. In car parks such as you describe it is usual for the notices to comply with the guidelines.
The matter of enforcement is another issue. Skyline is quite correct in that this is not a fine but an invoice for services. As such it must be pursued through the County Court if it remains unpaid. paraffin�s contention that to ignore it is a likely route to success is fraught with danger. The companies that run these operations are quite large affairs and it is not in their interests to let it become known that if you ignore their enforcement notices they will simply go away. The charges will probably increase if you do not pay early (usually within 14 days). The costs of pursuing a claim through the courts are minimal and, if you lose, will be awarded against you anyway.
It is a popular misconception that these parking enforcement organisations are cowboy affairs operating on the fringes of the law. They are usually well run businesses that know the rules by which they are bound and they will contest cases where payment is not forthcoming. Once judgement is passed bailiffs can be employed to recover the debt and with a County Court judgement against you, you may have difficulty with banking and finance. Do you really want all that for the sake of ninety quid?
My advice is to learn an expensive lesson, pay up and move on. The potential aggravation for you if you ignore the notice is likely to be far greater than for the parking company.