Thanks for the tips folks. It seems that castors/wheels designed to fit square section leg light stands easy to find in the US, but less so in the UK.
I reckon that wheels designed for 30mm round tubular legs should do it, as long as there is a locking screw. (May need a bit of shimming.)
Edit. Or, of course, get some metal tubing/rod which will slide into my square section legs and get wheels to fit that.