Bongo Fury

Hmm thanks! I bought 3 spare ones so might have to try again, would any of the other components in the transister pack be able to cause such an issue?

I’m just glad we have heating again!

Well, not by any fault that I've come across to date. The major stress for the transistor is heating and any voltage spikes created by the motor, which a poor or intermittent earth could easily cause. I would look to check the transistor in situ and see if its responding as expected, between different settings, however it isn't easy to test in that circuit, otherwise its check for leakage alone.