I now suspect that this problem is simply due to the chipset not being supported in this version of the X server. My comments about the server working, but then locking up, were probably in reference to an experimental version (4.3.0) of the X server that I'd installed (and then forgotten about!) precisely because it was the only version I could find that *did* support my chipset.
Bug 229628 contains details about the experimental version locking up.
