take the clamp off completely, then use summat thin like a watchmaker screwdriver to clean all the crud out of the lip all round the base of the dizzy. drown it in proper penatrating oil and leave it for an hour. come back and try again, if it still wont come off clamp some mole grips round the shaft, and gently tap them with a hammer back & forth. not looking to use brute force here, just gentle but sharp taps to shock it loose.
