I would wish you luck Andy, but unfortunately this is what is known as an “absolute offence”.
This means that the Police only have to prove that you were in a restricted area to obtain a conviction, and your reasons for being there are irrelevant.
Don’t get me wrong- I’m on your side but the law is clear on absolute offences.
When I strayed into a 7.5 tonne limit at Usk and was stopped (by two of the nicest coppers you could ever wish to meet), my employer paid the fine. Have you approached yours?
Anyway, some good news. It’s not endorseable.
Vince