OK, so I took the car to a different shop, and had it put on a Hunter Hawkeye rack.
They were able to put the rear in spec for me, however the front was another matter.
The front right was able to be set to the correct camber easily enough, however on the drivers side, the closes they could get it was -1.16deg.
Additionally, the camber on both sides was ~1.5 degrees.
They've set the passenger side to match the drivers side so at the moment I've got ~1 degree negative camber on the front.
I'm positive that the guys working on the rack are competent as we spent a good while talking about it and I trust them.
I think the issue with the camber on the drivers side is down to the upper arm pivot; I had tried to replace the upper arm bushes on that side before I took the car off the road a few years ago and didn't manage to get the pivot bolt out. I suspect I made the problem a lot worse beating on the pivot and I reckon it's somewhat bent up now, pulling the arm in/up from where it should be affecting the camber as a result.
I'm going to have another crack at getting the pivot out now that I have more of the right tools, more experience, and more determination. I think I can resolve this one
On the passenger side though, I'm pretty certain that the bushing is in good shape (when I replaced the upper ball joint on that side a few weeks ago, the arm showed resistance and elasticity bouncing back to its neutral position after moving it around). Other than replacing that upper joint and the outer rod end, I've not had the passenger side apart since I owned the car. This concerns me a little, as I can't imagine how the caster angle would be so far off spec, and consistent on both sides, without having been set delberately. I haven't looked at the position of the washers yet, but I'm hoping that at some point a PO had replaced the upper arm bushes and put the washers back on the wrong end.
Can caster go out of spec to that degree naturally over time? Is there any other issue which could cause such a big, but consistent, change?
I think the caster (and perhaps in a small way the camber) is the main reason for the excessive bump steer - it certainly appears that the tie rods have a downward inclination compared to the lower arms, which would cause the effect I saw at the first alignment shop.
My only other thought there might have been that I might have got the wrong outer rod ends, with longer studs than they should have; does anyone know if rod ends meant for the rear would fit in the front but with the wrong geometry?