OK, one has arrived! Not sure who to thanks as I know I hedged my bets and had two sent just in case I needed to modify one and it went terribly wrong - so thankyou to both senders!
After a gentle push it now fits perfectly - you could even say it was made for the job. Will post up the rundown of my now complete volcano stove brew kit shortly after I've fitted a knob to the lid.
Excellent!
Cheers
Mike
[EDIT] Found a note - Capt.Dunc got there first! Thanks chap.