Thanks for the help guys, I found a post from sparkywig:
Just happened to check Bentley, brakes & wheels section 8, page 35.
"The 1968 through 1970 models are equipped with 5JK x 14 rims. All later models have 5 1/2JK x 14 rims. The 5JK x 14 wheels have an offset of 47.5mm (1 7/8"), while the later 5 1/2JK x 14 wheels have an offset of 41.0mm (1 5/8")."
so with that, and aspro's example, I should be able to work things out. On the practical front, I will ask Graham what size tyres he is running on Billy, and what the clearance looks like.
Thanks again,
Elliott.