Nice of your brother to be so lenient on you but he has a his job to do have you got a black and silver plate fitted as I have seen later bikes with them on I want to put one on my bike but do not want to be pulled by police for it.
I have a black and silver smaller one on the Enfield and never had a problem with Plod. It passed the MOT with a yellow one but of smaller size than standard.
Fix the black and silver plate over the yellow one and then if Plod stops you apologise and say that its your show plate and you forgot to take it off.
If I remember correctly it's 1973 change over year from back/silver to yellow/white as anything on an 'L' could be either but 1973/4 'M' and onwards always had to be yellow/white.
Ah now that's an interesting one as we're both right. If you read that link it say that since 'April 2015' looks like they've updated and moved the goal posts!!
I'm really interested about how to map my bike? What things do I need and is it expensive to buy what I need? I took mine for a service recently and it doesn't sound anywhere near as lovely as it used to Many thanks