Hi DazTopp. I am using a 42 tooth chain ring combined with a Shimano Alivio HG400 MTB 12-36t:12-14-16-18-21-24-28-32-36 cassette. I got the cassette from chain reaction and the chain ring from the shop where I bought my bike, ebikeshop.co.uk in Farnham, Surrey. The name on the chain ring is blackspire if that helps.