An update: After lots of 3-in-1 and some percussive maintenance with a hammer, I managed to shift the old pin
I got the new peg in place, but the pin for that, instead of fastening with a split-pin has a circlip, except it currently doesn't protrude enough from the bottom to get access to the groove it's supposed to fit in.
So tomorrow I'm probably going to do a little judicious work with the Dremel to deepen the dip the pin goes into which should sort it (fingers crossed!)
Oh, and I'm going to put copper slip on the new pin too, just in case I ever have to take it out in the future...