@mickR This is correct. The plate does conform, and was issued by a company registered to produce plates and has the relevant BS watermark on it. The issue isn't/shouldn't be the plate itself, but I expect it's down to the visibility.
It goes against my soul to pay up for a fine without evidence. I am however on the fence with regards to does it bother me enough to want to pay more than £100 to prove a point. I suspect I will end up just paying it (even if it's just to keep the wife happy). Blah.