Righty... the situation's changed a bit now, as Brett outlined. Raid is now repaired, but won't start on boot, for reasons not known.
Syncing doesn't cause lockups, or that's not proven, but something does cause a USB devices to stop responding, eg keyboard & mouse. this happened on the live cd as well, although the PC was working normally otherwise.
Basically, as it stands now, I have a working raid, in the live cd mdadm --assemble --scan /dev/md0 works perfectly, but something has changed that stops it being loaded at boot time; the kernel gives up waiting for the root device to be ready & drops me at busybox.
I could make a backup of the array and reinstall the whole system, but I'd prefer to solve the problem if possible. I'm making a full backup tomorrow anyway, whatever happens.