I finally got the seal out, my first attempt went a bit shit, I was drilling the seal and screwing in a wood screw to give me something to pull on but the screw kept pulling through with no movement of the seal, then the drill bit snapped so I left it hoping it would fall out by itself. Youtube videos only seem to show removal of cranks seals that were installed 5 mins before and not 35 years ago. I gave it another crack this morning after getting home from work, something about staying up all night really numbs your brain and helps you not to overthink things so 2 mins and a small right angled pick through one of the drilled holes helped me lever it out from behind. Yeah
I need to clean up the seat then can bang the new one in. Once it's in I'll do another vac test and see what the mag side seal is like.