Wasn't going to post on this thread again till I had sorted a few things out, but the way I'm going that could be a while. So...
a bit of progress, with a caveat.

The vintage Swarovski scope I fitted to the first Ross sporter is image-moving (I think!) but is not perfectly aligned with the bore so the exit pupil is not centered.
This means I have to align the crosshairs with the target while simultaneously trying to centre the exit pupil, and if I lose concentration I get a flyer, parallax error I assume?

Anyway, long story short, I need to shim the scope to bore alignment, or mount a modern scope for further accuracy testing.

Meanwhile, I pulled the target in to 50 metres to make it easier to concentrate on the sight picture and made the following 3-shot group:

Finished by ringing the 500m gong by holding a couple of feet over.

So, its looking good thus far, but I need to sort out the scope issue. That vintage Swarovski does look the part, so it will be worth the effort to mount it correctly.