I had no idea that could be a problem, but we'd already removed all the text boxes but one, off the theme layer, and we were running out of ideas so I just deleted that gobbet of code, compiled and refreshed my page and *\o/*. Hopefully, I'll remember for next time, if the problem ever occurs again.

Not that it's likely to now, not with the way they've added all that extra functionality to the customise area.