Right, this is a new computer, so I'm still not sure what's what in it.
The computer came with an AVerMedia DVB-T/PAL tv tuner card. (scroll down to the tv tuner section on this page: HP Pavilion p6565uk Desktop PC Product Specifications
One thing that I-haven't-got-a-clue-what-it-is is some small thing that looks like an antenna (attachment 1), merely connected to the 'ANT' socket on a different (don't know what) card on the back.
The second image is basically my question (along with 'what on earth is the antenna thing??')
(I tried getting WMC to detect a TV signal with all cables as pictured, but it said it couldn't detect any (that could be because the TV signal in this house isn't particularly good, but I'll probably be back in Salisbury in a fortnight)
(yes, the cables round the back are a mess. Don't worry, that's just a temporary position)
EDIT: I looked inside, and the unknown 'ANT' connection isn't a card, it's a cover with the socket that then has a cable leading to something plugged into the mobo)