Gaming PC (new build) ~£1000

Intel i5-750 http://www.ebuyer.com/product/194854 £156

Asus P7P55D-E Pro http://www.ebuyer.com/product/194854 £162

G-Skill 4GB (2x2GB) DDR3 1600MHz/PC3-12800 Ripjaw Memory Kit CL7(7-8-7-24) 1.65V http://www.ebuyer.com/product/180490 £106

Sapphire HD 5870 1GB http://www.ebuyer.com/product/199644 £320

Seagate Barracuda 7200.12 500GB http://www.ebuyer.com/product/158860 £37

Antec Truepower New TP-550 http://www.ebuyer.com/product/159918 £70
Should also be enough for a single 5870. If you do want a power supply can power two 5870s, then get this: Seasonic S12D-750 £93

Cooler Master HAF 922 http://www.ebuyer.com/product/164961 £83
Unless you have your eye on something else.

Samsung SH-S223 22X DVD±RW/RAM/DL Serial ATA Black Bare Drive - OEM http://www.ebuyer.com/product/145450 £15

Total: £950

Ebuyer don't really have any worthwhile heatsinks atm, so you might have to get one from somewhere else if you do more than a mild overclock or want to keep temps/noise down. These are two that I'd recommend:
Titan Fenrir V2 £35
Prolimatech Megahalem Rev B £45 - Will need to buy fans for this one.
You will need some thermal paste as well, something like this: Arctic Silver 5