Try this one for size:
PCPartPicker part list / Price breakdown by merchant / Benchmarks
CPU: Intel Core i5-3470 3.2GHz Quad-Core Processor (£143.99 @ Aria PC)
Motherboard: ASRock B75 PRO3 ATX LGA1155 Motherboard (£60.00 @ Amazon UK)
Memory: G.Skill Value 4GB (1 x 4GB) DDR3-1600 Memory (£25.99 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£50.00 @ CCL Computers)
Video Card: MSI GeForce GTX 650 Ti Boost 2GB Video Card (£144.69 @ Dabs)
Case: NZXT Source 210 Elite (White) ATX Mid Tower Case (£38.65 @ Ebuyer)
Power Supply: XFX ProSeries 450W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£35.92 @ Amazon UK)
Optical Drive: Samsung SH-222BB/BEBE DVD/CD Writer (£15.95 @ Amazon UK)
Total: £515.19
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-04-13 21:00 BST+0100)
Slightly over £500, but what do other people think?