Thanks for the ideas guys, you were right Tim - if in doubt get a bigger hammer
... last night I managed to borrow a big hide mallet and that did the trick after a few whacks around the cover (it was explained to me that the rubber mallet 'bounces' off the case, whereas the hide mallet almost 'stops dead' on contact with the case and so passes the force onto the case instead of kicking back), case was held on with threebond type stuff (no paper gasket). Checked all the all the plates and they were fine and the basket had no groves in it as well, so no problems there. Off to Baines later to pickup a paper gasket to make it easier in future. As for the unusal clutch noise I think I need to compare mine to others at the same time, in case I'm being paranoid