How do I remove the pully from my B/S mower engine?

I want to change the drive belt of a Castelgarden twincut ride on mower - there are guides round the pulley under the engine deck and if I can get the pulley off (away from the guides which don't seem to be removable) I can refit the belt. But the nut is very tight; there are bolt holes in the pulley but bolts fitted to stop it turning just bend, and I don't want to force things too much - is it even a left hand thread - anyone had this before??