Thanks Billy for the info. I have my local sewing machine dealer on the lookout for needles for me - I have a couple of hand cranks and an old Jones which take round shank needles. He had plenty of treadle belts on hand, so I don't need to worry about getting those.
My main concern is the very odd tension device on the Class 15. No matter how I put it together, it 'unthreads' after a short time. I think I will have to stop fiddling about with it and get someone with more knowledge take a look at it. It's frustrating cause when it works, it sews beautifully.