This has apparently just been fixed after changing the theme again and now works whichever theme I have. Since I can't delete the post and for those interested, what happened was...
Something really wierd has happened! I've been trying to edit the theme using Regulus and the text has gone tiny (too small to read). The only custom HTML I entered was in the sidebar "text" widget, which I've now removed. The text however is still tiny, and I mean tiny!
Can anyone see what's wrong wtih it? I tried changing to another theme that cannot be customised but the problem remained.