Thanks Guys.
Just to let you know that I managed it in the end - more Plusgas over the weekend, tonight I heated up the nut in boiling water, into a vice and a little leverage help from a tight fitting bar in the throat of the carb. Off it came.
I'm glad I persisted, the old gasket looks to be in a poor way. I'll make sure that the new one from Amal fits as it should.
Thanks again for your advice, Simon