A friend tipped me off there was a problem making a guestbook entry on this site. I looked into it, and sure enough, it wasn’t possible to make any comments anywhere.
I don’t know why this happened. I think it was probably something to do with alterations I made to the former theme.
I’ve now installed the Digg 3 Columns theme and everything seems to be okay.
This theme is very neat and I’ll stick with it for a while. The only downside I’ve found so far is that previously I could size photos larger, but now they stretch across the sidebar.