You'll also probably find that a "mark" has been recorded against your bike and any ANPR activity will be triggered as previously having an illegal number plate....that alone may warrant a stop check.
If you can clearly see the plate and read the VRM with ease at 57 feet, then challenge it. Its not in the publics interest to prosecute you if its lawfully legible.
IMHO they may have stopped you hoping to find something else wrong, if you have a dodgy plate you may also have a dodgy exhaust, tax, insurance etc. etc.
Personally I just have a huge standard plate so its one less thing that they can pull me for.
Thtargetsad condemnation Steve- the likelyhood of you getting pulled will depend on how well the bobby has done with his figures that month I had a 51 plate FZS6 with a standard sized plate with standard lettering which did not have the BS AU number on it and my bike failed its MOT- had to find the cracked and broken one and stick it together with tape and take it back to the MOT station
Solution: Replace with legal size plate. Lesson learned.It may seem petty, but it's the law. If it had been a van towing away your bike after stealing it, and someone said...I couldn't quite see the plate as it was so small....
why not just let me off with a warning instead of pushing through with the fixed penalty