It has two caps, a start and a run. As it still satrts but runs slowly I'm guessing the start cap is OK, and the run cap is faulty, as from what I've read faulty run caps make the motor run slow and overheat, which is my problem exactly. I have the values for both, and I did read how you work out which was which, once I read that again I'll take it out and have a look at it. I'm hoping it has a discharge resistor on it so I don't electrocute myself.....
I also read that capacitors can go out of range over time so I'm hoping that's what has happened.
I did phone Clarke Technical, and as nice as the bloke was, trying to be helpful, he didn't really know as much as I did already, which wasn't a lot.