Quote: "On a side note I am tempted to invest maybe 100 GBP into facebook adverts, anyone really got any experience / other ideas?"
Facebook is an awesome advertising platform. I use it to advertise stuff locally too! It's great. I had a garage sale, cost me one USD to advertise on facebook, and I had one confirmed visitor who told me he saw my ad on facebook, he bought 3 dollars worth of stuff

Not a great ROI, but I was trying to get rid of stuff
Of course, advertising nationally, but still to the right people, can yield good results. I live in a town of only 10,000, many of whom are not on facebook.
With Google and Facebook, you can easily find coupons for $50 to $100 for opening accounts. Just search around. Shoemoney.com often advertises these types of deals, and he offers great insight into online marketing in general.
In the newsletter there was mention of an nvidia store, you would want to certainly get in on that.
And some important advice, make absolutely certain your game is as professional as possible. Make sure it complies with all the "games for windows live" rules. Even if you don't pay to get certified, nobody's going to like your game if it doesn't follow the basics like where to store save files and how the installer should work, and xbox controller support (when applicable).
Build hype, lots of hype. How you do that is up to you. One company I recall bought pizza for google employees.
Also, don't be afraid to invest in good artwork. For a few hundred bucks, you could get a college student majoring in art to help you. He'd be happy to get some spending cash and to add a real project (that is, if the game's any good to begin with) to his portfolio, and for anybody who has a full time job, it doesn't break the bank.