Hi all, thanks very much for the replies.
Issue solved now. The spokes we have are the same dimensions as those sold by the well-known Bultaco shop (there was a mix up on the dimensions they gave).
After looking at a photo of a whole wheel assembly for sale on eBay, we saw that the wheel was laced with a 2-cross pattern on both sides. Lacing this way brought the offset on the rear wheel right back to where it should be. It's all built back up now, and no issues.
The front wheel is however still offset to the non-brake side. It's laced 3-cross both sides. It needs to come about 15mm across.I'm not sure if we can pull the offset by retensioning the spokes, or if it's perhaps worth trying re-lacing with a 2-cross pattern on the non-brake side.
We've fitted a new Mikuni carb at the same time, and it's made such a difference. Choke on, one kick, a wiff of throttle and it's running. With the Bing fitted, it would be all sorted of combinations of kicking, choke on, choke off, throttle, no throttle etc etc. It has been satisfying tackling one job at a time and feeling the bike improve with every step.
Thanks again,
Tom