Daryl
I have a 10 x 560 belt from the idler to the spindle. A 10 x 584 belt from the motor to the idler and a 10 x 914 from the motor to the spindle. These may not be the original sizes as I bought it 2nd hand and sourced my own belts, but they work.
You may like to see the modifications I have made to my Clarke CL500M over the years. It certainly makes it a more versatile lathe and I have no complaints.
http://www.bsa-c15.org.uk/engineering/clarke_cl500m_modifications.htm
Cheers Cedric