Dave,
apparent contradiction between " I can not call anyone a liar" and "no-one has ever seen them except a well known Mudcat liar" then?

No contradiction intended. I do not accuse Jim of lying about this, he is just wrong, but he does tell lies.

Anyone could write such a notice anytime so I would not claim none exist, but that could well be the case.
They are illegal so it would just take an anonymous call to the police to get one removed and the perpetrator in a heap of trouble.

I do claim, with great confidence, that they are not "common throughout Britain" or one of us living here would have seen one.
We still haven't Jim.