I dunno, 40Gb isn't a small amount... Assuming your PC is reasonably recent Stu, that's about 10-20% of your hard drive's total capacity.
I have my BB supplied by way of PlusNet, and I'm on their 60Gb a month package, cos 20Gb a month I knew wouldn't be enough. I don't know just how much overhead Xbox Live generates, but unless you're watching videos on YouTube pretty much constantly, 40Gb a month should be plenty. Forum, social sites, general browsing and stuff really doesn't use much bandwidth and if you don't go in for downloading films or music, I think you'll be fine. It *IS* worth checking what you'll be charged should you go over this though, just to be on the safe side.
My usage tends to come in at about 35Gb a month, which covers general browsing, FOG, social stuff, comics like Dilbert that I read, an assortment of podcasts that I listen to, BBC Iplayer & YouTube, iTunes app downloads, film clips, game patches, even complete operating systems sometimes. Up to you really.
It may be worth calling Sky's bluff on this, threatening to cancel your subscription and see what happens. Say you want the same deal that they're offering to new subscribers, perhaps, or just carry on as you are at the new address..
Works with mobile phone companies, so you never say yes to the first offer they make.
---Edit---
Maybe ask the people on the end of the phone to look at your qaccount, and see how much you've actually used. Any halfway decent ISP should be able to say "Yes Mr Bloggs, this month you've used XXX amount, last month was XXX, and the month before that was XXX"