Graham, definitely a right hand thread, no ifs or buts, so when trying to loosen it you have to go counter clock wise when looking straight at it.(Obviously for safety reasons you'll have the spark plug lead disconnected). They can be real stubborn at times and hitting it wont get it loose either. If that is the case, load the mower in the boot and go down to your local mechanic or tyre shop and ask them to crack it off with their rattle gun.
Good luck!
