Well, I think I’ve got around the comments problem. It was something to do with Spam-Karma so I’ve deactivated it and taken the opportunity to upgrade my WordPress installation, the latest version of which includes the Akismet anti-spam utility. Depending on how that performs, I may get the newest Spam-Karma as well (assuming they will work together ok).
If anyone sees any problems with the site over the next few days, do please let me know via the comments or by e-mail garry(AT)trickylight(DOT)co(DOT)uk.