Attachment.php does nothing

I’ve been adding images to a few posts using the upload function. This gives a working thumbnail, but if you click it (with the link set to ‘page’), it takes you to
https://…/category/post/image_title/
(on my website, this is something like https://www.thezahir.co.uk/politics/the-arms-trade-death-for-profit-or-a-fair-deal/soliders-in-the-balkan-conflict/)

There is an attachment.php file in the theme (a custom theme, but the attachment.php is taken straight from the default theme) but when you click an image it just takes you straight back to the index template. This seems…really odd. Any advice?
(Go check it out if you want, https://www.thezahir.co.uk)

Oh, this is on wordpress 2.2.3 (but I’ll be upgrading to 2.3 soon, if that will make any difference to this issue?)