I'll be purchasing this tomorrow if no-one else has any suggestions:
Processor: Intel Core 2 Quad Q6600 £137.95
Motherboard: ASUS P5KC AiLifestyle Series £73.13
Memory: OCZ 4GB (2x2GB) DDR2 800MHz £69.99
Graphics Card: Point Of View 8800GT 512MB £119.98
Hard Drive: Already got (Seagate Barracuda 500GB SATA 7200.9) £0
DVD/CD Drive: Pioneer DVR-215BK 20X SATA DVD±RW £17.99
Power Supply: Corsair VX 550W PSU £53.63
Operating System: Microsoft Windows Vista Home Premium £62.96
Cooling Fan: Arctic Cooling AC-FRZ-7P £15.24
Case: I-Cute Black UL01 Mid Tower Case £20.99
Front Case Fan: Hiper 120mm Case Fan £5.99
Rear Case Fan: Hiper 120mm Case Fan £5.99 Total: £583.84