It's a large, printed notice, in capital letters and underlined. Whether or not that constitutes "a polite notice" - well....... My view of a message presented in this way is that it indicates a serious intention - but, I guess, that's subjective! I suppose my fear is that next time, I'll get a bloke waiting for me, with some not very pleasant words - or even getting blocked in, with the logic being that "I'd been warned".
Oops, forgot your second question: no, they are not purporting to fine me.
I don't understand the part of your answer that "Parking there without permission is a trespass", given that the users of the various businesses (including the gym) park without any specific permission, e.g. there is no parking payment. It therefore seems to me that anyone parking on the estate could be open to a "claim" being brought against them.... ?