This happened to my wife last year.

She got stopped for an incorrectly spaced number plate.
Then realised that her MOT was out of date (obviously my fault). We got the car MOT'd and then went to the police station where they read her her rights again for not having an upto date MOT certificate, but noted that it was an oversite and that the car was now MOt'd. We were told to wait for note of prosecution/fine. Waited over 6 months now and have heard nothing. I don't think now we ever will.
Cue it dropping through the door any time soon.