I couldn’t get my verniers inside the rim of the new wheels to measure the hole size, so Steve Law at Speedshack very kindly put a couple of second-hand ones in the post to me so that I could check for sure … they’re 5/8″ it turns out.
Excellent service from Speedshack 
Now comes the question of refurbishing them 
I’m guessing that media blasting would be the quickest/simplest way of removing the old paint, or could I use Nitromors? 
I’ll have a bit of a google and see if it’s known for eating alloy…