I got several more recommendations and suggestions about issue trackers. I updated my master list with these new suggestions:

• PloneCollector-NG would be a sensible choice if I had Plone deployed.
• Issue Tracker. Had several pointers to this with recommendations. Came out of Red Hat.
• ASP.NET Issue Tracker. Actually example ASP.NET code, so unsure of licensing.
• Whups. PHP-based, with web, command-line and XML-RPC interface. Seems part of the larger Horde project, and restricted to the features needed by same.
• BATTS. Old-school command line and email tracker. Perl-based.
• EZ-Ticket. Early stage PHP/MySQL system. Looks highly trouble-ticket oriented.
• GNATS. Another old-school style bug tracker. Written in C and very mature. Worth a look simply because it's been around so long, but I can't find evidence of deployment as broad as for some other well-known trackers.
• Eventum. Used and written by the MySQL guys. I had two recommendations for this. A first glance shows a useful feature list and uncluttered interface. Has CVS integration.
• OpenPSA Support. Part of a suite of software for professional services organisations. Not suitable for software bug tracking, by the looks of it.

Out of these, I'm going to add Issue Tracker and Eventum on to my list of trackers to trial.