650W will be enough for a system with two GTX 560 Tis. Only if overclocking extensively or if you have aspirations or want the possibility/option to upgrade to two more power hungry graphics cards would more than 650W be justified imo.
The new one is £85. While it is definitely a good unit, I'd question whether it is worth £10 more than this unit which has the advantage of being modular:
Adata HM 750W 80Plus Bronze Modular £75 http://www.ebuyer.com/product/261367
Corsair 650TX - with that you'd be able to run a 580 with headroom, so it's definitely fine for what you have, you could go lower but 550W could be a bit limiting, especially in regards to future upgrades.
Cool thanks lads, I decided to go for the Corsair 950W TX Series PSU - 78A 12V Rail and just spalsh out on the cash, £120 but I guess it will last me a few upgrades and for when I SLI the graphics card.