what I do is remove the hold down clamp completly, then scrape out all the crud from around the base and in the lip around the dizzy with a thin screwdriver. then drown it in proper penatrating oil (not WD40!) and leave it for an hour or so. Then come back and clamp some mole grips round the shaft, if you cant free it by hand tap the grips gently with a hammer, this will shock it loose.