Thanks Brian, all.
I think I've got this sorted out now... just had to ratchet strap it all together to get enough thread to get all the nuts on it.
Upper pivots on mine are 260mm or 10.25 inches approx
Lowers are 270mm or 10.675
The uppers seem to be just long enough and no more, for their application, whereas there's a little more thread available in the bottoms. I did the bottom ones first so maybe I'll try a swap around and see how that looks, but I'll do some more measuring first, as they're a bit of a pain to change
I'm not sure if that's normal, because I've got rose-jointed adjustable wishbones at the front. Not sure if the rose joints and spacers make the pin length requirement longer, shorter, or about the same.
I suspect that removable pins increase the number of servicing options (you can remove it in either direction).