If he stopped you when he was in plain clothes he wasn't acting unlawfully... but it wouldn't have been an offence for you to fail to stop for him.
Can you be more precise about what happened?
If he's going to follow it up there should be a record. This can either be in a pocketbook, computerised record, report book, intel report etc. You will (generally) have a right to see this but depending on the situation and the timings, you may have to wait until proceedings are finished, and if no further action is taken you may have to apply through Data Protection regulations.