Re:
Sorted!
Well, in theory at least. I've proved my theory, now just need to get another stem....
I found one of those very thin scuff washer type things, and thinking I was being all clever, put that in against the bearing race cover, under the stem, thinking that would deform a little if necessary if the stem wasn't quite flush in all places. No difference at all.
At this point, I left the whole bike in a very cold workshop, gave it a good talking to and a damn good thrashing with a large branch, and came back in, in a strop. As I calmed down a bit with a cider, I started thinking, what I actually need is another stem, in case it's that that's really warped on the base or something. Didn't think I had another one, until I saw the NOS one that the frame came with looking at me from under the chair!
So I went back out with that and tried it instead, and it's a lot better - still a tight spot, but at the front now, and not so bad. As that stem is far too long, I need to get another anyway, and at the same time I'll get one with a far lower 'stack height', if you see what I mean, as both the ones I've been trying are very deep (a bit like me

) as in, 50mm, so I wonder if a shallower stack height and more spacers instead would make the whole thing more pliable.
As I say, wonderful theory, and tomorrow I should have access to a few options if I can get over my mate's bike shop for a few hours. He'll probably happily swap the stem I was trying for something more suited anyway, so we'll see. Might be able to sort it Sunday then....