I'm pleased to report that following much helpful advice and the loan of a 41mm socket from Forum members I was successful in removing the crank nut on my engine without having to drill and chisel it off.

Surprisingly (and no one was more surprised than me) I was able to remove it without any difficulty with a Machine Mart 60 quid mains-powered impact wrench. There was no need to lock the crank or anything else - it was completely without drama and took about thirty seconds.

If anyone is buying a 41mm socket - for your information it needs to have a maximum OD of 56mm - preferably a mm or so less - in order to fit into the crank pulley recess.

Peter