Well, progress has been made. I couldn’t remove the plug as one of the bolt heads sheared off. I will try and remove it with a stud extractor later on, and then replace the three machine screws (if I can find replacements)
I decided to try and remove the spring pin in situ, and it actually came out quite easily (just as Colin had said). There are some deposits on it, but it is not rusty so must be of stainless construction. The sheaves are quite burred so I will replace them and hope for a good result. The pin, once removed, is a loose fit through the sheaves which were actually stuck against the sides of the mast plug so I will clean the inside of this with some wet and dry and then oil.The mast plug is alloy.
Thanks again, Seb