Hi Andy
I'm sure you've already found the answer since you posted this nearly twelve months ago but this place isn't too far away from you and Steve is a really helpful bloke
http://progressiveclassicproducts.co.uk/frames.html
Its a nightmare. I just took my old Lambretta for MOT and had to take the slightly smaller, 6 1/4 inch number plate off for an hour and put a full size one on.