Thank you. Managed to get the nut off in the end. I think the issue was it had been done up by some sort of ape.
I have a new starter that has been sat in the shed for a while. Needed to replace it last year but had the same problem so replaced the bushes in situ. As I had a new starter, I undid the long 10mm bolts that run along the starter and got as much of it out of the way as possible.
I now have a gearbox that can separate from the engine but doesn't want to. Going to have to put an engine hoist in through the door to take the weight I think.
Is there anyone near Godalming, Surrey that has a hoist or experience I can borrow/exchange for a beer?
Thanks again all. My Landy love is fading but sure she'll win me back again!