Well, you're supposed to have it displayed at all times on the left side preferably at the front of the machine....
but.... as you're all aware, they get nicked even with lockable tax disc holders on sometimes (don't they psychlist?), so upon speaking with policeman, we can carry them on ourselves at all times, displaying a note in the holder saying that's where it is...
IF, we are not with our bikes, they can always do the check from DVLC which will confirm we have paid, which is all they're worried about...
