In earlier post I mentioned I had foolishly damaged the teeth on the double sprocket that screws on to the end of the cylinder shaft in my efforts to remove it. I have sourced a (genuine Morrison) replacement sprocket from a dealer in Australia which arrived yesterday. It looks identical to the one I'm replacing. However when I screw it onto the shaft it becomes very stiff after a couple of turns and I have come to the conclusion the thread pitch is slightly different.
My options seem to be to force it on - effectively recutting the threads using the shaft; to take it somewhere and get it re-tapped with the right thread (NB it's left hand thread); or abandon my investment in this new part and seek some other solution. The part cost around £60 so you can imagine the last option is not attractive. Apart from this I have also refurbished the rest of the machine and had the cylinder and blade re-sharpened.