So it turns out it wasn’t anything I did re: the change of themes, it’s a small problem with the Mandigo theme not linking to the theme options page in WordPress v3 correctly. I can work around it for the time being, although I am hankering for a new look for the site (but still want to keep / use some of the header images).
The new default theme is okay but it’s all so big and I much prefer small sites. I guess I can edit the theme options and have a play.
Anyway, I finished my little plastic knight, and it’s not a half bad job considering.