So I seem to have fixed it. I was up a bit earlier than necessary this morning (my sleep pattern is all over the place due to having a baby in the house) so thought I’d have a look inside it.
I decided to lop a bit off the top of the insulator. I couldn’t seem to get it off the pin, so just attacked it, about 1mm or so from the top, with the wire snippers. It came off, but not exactly straight (Once this bit was off, I was able to get the insulator to slide off as well).
I screwed it back together, and it turned out I’d taken too much off. The pin was going further into the 510 and I couldn’t keep screwing it with pliers.
So back apart again, and I put the removed piece back. Because it wasn’t a clean cut, it didn’t fit too flush and sat a little higher. It seems this small difference has been enough to allow work. It’s now pretty tightly screwed on and the airflow holes are perfectly aligned. No movement in he positive post. And it’s working fine.
I think a lot of luck was involved, so wouldn’t necessarily recommend doing this, but I would say that the aim is to add to, instead of remove from, the height of the insulator, if anybody is going to try it.