Thanks for the responses, and apologies if i'm in the wrong group,
it's difficult to know where to post!

I do think it's powered... in a Homer Simpson-esque way i put my
finger on the wire several times and got a buzz each time before
working out that their may be power there. The other thing that makes
me suspect this is that other homes on the development (that opted for
the camera install) appear to only have one wire going to their
cameras.

The charge for the fully fitted camera from the home builder was about
£200 hence the reason I decided against it and am now looking to see
if I just need to buy a cheapo camera!