Hi gb2world, thanks for explaining that, what a fool I am! Talk about not taking my own advice - I've been developing three sites for the same client, and hadn't even used my own, new version of the shop_closed.html on this one!
I've put the extra forward slash in now and that's fixed the problem. Am I right in thinking that that now makes it an absolute, rather than relative, URL? So from the
http://www.tnplshop.co.uk/Card-Covers/
page, it was looking for
http://www.tnplshop.co.uk/Card-Covers/skin1/images/etc.
(which obviously doesn't exist)
instead of
http://www.tnplshop.co.uk/skin1/images/etc.