There are a few arguments you can run here - firstly, there is no consideration offered by that sign (it doesn't offer you the opportunity to park on certain terms, all it does is forbid) - on that basis, no contract can be formed, and without a contract, no money is owed.
Secondly, if the vehicle was only stopped for a few seconds, there's a fair argument to be made that the driver was not presented with a suitable opportunity to read the terms and conditions on the sign. A driver must be given the opportunity to acquaint himself with the terms and conditions on offer before he can be bound by them... Even if the signage could form a contract (which you will deny), the driver was not given sufficient time to consider the signage.
Another point to raise: what is your relationship to the vehicle? The wording of the notice suggests you are not the registered keeper, is this a financed/leased vehicle?