Update: the QCad community edition was a bit glitchy so I bought the current commercial edition (GBP 23) which turned out to be 32-bit only so I run it in a VirtualBox VM. I also built
LibreCAD, a FOSS port of QCad community edition, heading away from Qt3 toward Qt4 and cleaning up the code. Usable if not totally stable -- and more convenient than using the VM.