Well from google, I found a site map: http://www.next.co.uk/sitemap.asp
, towards the bottom it has a 'MY ACCOUNT' thing, where its got an option to pay.
It looks like (just going from what I saw when I googled it just now) They just charge things to your account, and charge interest? when you don't pay. (it looked like 30%, ouch!) I don't know how they can charge you with no Credit Card info, but eh. I'm not from that site, I have no idea ;D