Bazza,
The tapers on crankshafts and flywheels need to match perfectly if they sre not to work loose. If the flywheel is also equipped with a key, this should only locate the flywheel for tightening purposes - the taper should be doing the work of holding the flywheel. If you have a key fitted, remove it, apply a small amount of grinding paste to the taper and then lap in the flywheel. The end result should be a matt grey finish over the whole of the taper. When you have achieved this, remove all traces of grinding paste, refit the key and make sure the flywheel doesn't ride on the top of it. Then fit the nut, making sure it does not become thread bound before pinching the flywheel. With the nut fully tightened, you should not have any more problems with the flywheel coming loose.
Regards,
Eddie.