Are you prepared to risk being sued?
It would probably be about £100.
That is worst case scenario. They wouldn't get any more than that.
Is that OK?
In that case, just ignore them for the time being.
This is a private land fine. They are not unenforceable despite what is on the internet on the topic. They are just very difficult to enforce. It is not particularly cost effective for them to sue over one fine.
If they do sue then you can always put them to proof on the adequacy of the signs.
The sum of the fine has been considered in the case of Beavis and it has been held to be within reasonable limits. Actually a sum of £85 was considered to be a reasonable amount.
If they sue and succeed then as long as you pay within 28 days of judgment it will be removed from your credit rating.
Can I clarify anything for you?