As has been said these are not fines, parking companies are unable to issue fines they are penalty charges for not complying to their parking restrictions. There should have been easy to see notices on private land stating parking is limited to customers only, the maximum stay and that wheel clamping may be used (if this is the case) the law on wheel clamping on private land is being changed at the end of 2012.
If the penalty charge is not paid the registered keeper’s details may be purchased from the DVLA and a penalty notice sent to the registered keeper, if payment is still not made the parking company may huff and puff a lot over what they can legally do, but in reality all they can do is take you to the county court, which they sometimes, but rarely, do, they have in the past claimed breech of contract
If you pay or not is up to you but I strongly recommend you do not make any untrue statements.