I know this is a topic that's come up a fair bit but I thought I'd share what I've just been told. Whilst down my local riding school today their office lady noticed I had my 'L' plate attached to the back of my hi-vis jacket and informed me that by law this was illegal and that your 'L' plate needs to be attached to the vehicle. The penalty is 3 points and an £80 fine.
I know a lot of people will say they've been stopped by police and nothing has been mentioned but at the end of the day I think it's down to the policemans' discretion and they may not even be certain on this aspect of the law anyway.
Just some food for thought really as I know a lot of you have 'L' plates attached to bags, etc. I will be putting mine back on the bike, but mainly because I aim to do my test within the next couple of months.