New build

How about this one?
CCL Eclipse Hex Motherboard Bundle
Perfect for CoD Black Ops!
CPU: AMD Pheonm II X6 overclocked to 3.60GHz
RAM: 4GB DDR3 1600MHz Memory
MBD: Asus M4N68T-M LE Motherboard
COOLER: Titan Fenrir CPU Cooler - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 370
http://www.cclonline.com/product-info.asp?product_id=47...

CoolerMaster Cosmos S
ATX / 7 Expansion Slots
Compare CCL Code: CAS5041- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 149
http://www.cclonline.com/product-info.asp?product_id=20...

650W Corsair TX650W Power Supply
x 1 24 Pin + P8/P4
x 2 PCI-e 8 Pin
x 8 SATA
x 8 Molex
x 2 Floppy
80 PLUS Certified / 5 Year Warranty
Compare CCL Code: PSU0137- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 57.05
http://www.cclonline.com/product-info.asp?product_id=18...

Sapphire AMD ATi Radeon HD 6850 1024MB
New 6 Series AMD ATi Card From Sapphire!!
Engine Clock - 775MHz
Memory Clock - 1000MHz (4.0Gbps)
Compare CCL Code: VGA1449- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 153.49
http://www.cclonline.com/product-info.asp?product_id=58...

It's a little over what you asked for (about 20) but it should be a solid machine

I am a little uneasy about buying overclocked stuff.

Sorry for double post but just realized you posted a £149 case. Already have a case!  

Don't worry about the overclock. You can dial it back down just by entering bios and returning the multiplyer back to auto. But before you do that I would just download OCCT and check your temps while running your best whatevers. As long as it don't go over 62*C then there are no hazards to your equipment. The important thing is that it will run that fast if you want.  
I'm glad you already have a case cause now I/you can spend more on the video card. This one is a little better Sapphire AMD ATi Radeon HD 6870 1024MB - 21179-00-40R for 195.11 so we are actually under budget!