Update:
The mineral oil improved the clutch, but it was still dragging more than it should when it was hot. So I got a new set of friction plates and it seems to have fixed it, there's no trouble getting it into neutral now.
I also tried welding an extension on the lever at the engine end of the clutch cable and replaced the spring at that end as well, it made the clutch a easier to pull in and stopped me getting a sore hand. But I've just cut the extension off and re-welded it because of concerns of wearing the cable out where it exited the tube and also worrying that it wasn't giving the clutch enough movement when the bike was hot. I've got the stock spring on it again but think I'll have to do something as it feels a lot harder to pull in. Maybe just replace the stock return spring with a softer/differently attached one or weld a slightly smaller extension on the lever....
