These are invoices, not fines.
The law changed last October - before then you could say you couldn't remember who was driving and that would be the end of it. Now the registered keeper is liable even if he/she wasn't the driver.
There are three options.
1. Ignore the letters they send. They very rarely take people to court (civil).
2. Pay it.
3. Lie through your back teeth with the one get out clause I have found. Write to them and say the driver was one of your children who is under 18. Because they are a minor you are not bound to disclose their details. When you park in their car parks there are boards displayed which state that by parking there you have entered into a contract to obey their rules (i.e. don't park longer than stipulated). Under Contract Law, a minor cannot enter into any such contract and therefore their invoice is invalid.