In the Uk i believe they are as follows
grade 4 = half an inch or 12mm
grade 3 = 3/8 of an inch or 9mm
grade 2 = 1/4 of an inch or 6mm
grade 1 = 1/8 of an inch or 3mm
Good luck with your consultation JohnnyDrama Dr F is a good doctor. Please let us know how you get on.
Welcome to the forum Bullitnut.
On a side note it would be cool if you would start a thread about your own situation dude.