Ok, if they are a member you can appeal to POPLA
They offer a free, independent service and can examine your appeal for free.
If there were no signs and it was not marked your appeal will be upheld.
If it was the case you simply didn't see the signs, that is a different matter.
If they turn down your appeal you can still refuse to pay.
The Company would then have to take you Court. It would be a small claim and costs are limited.
The Judge can then decide whether you should be ordered to pay any fine and if so, how much.
That should resolve it for you.
Can I clarify anything for you about this today, please?
If you need anything further I am available for a follow up at no extra cost.