Okay, having taken on board some of the suggestions, i've added a motherboard and 4GB of RAM too and the price isn't too bad, still above £500 but not by a huge amount:
Intel Core 2 Quad Q6600 G0 SLACR, Kentsfield Core, Socket 775, 2.40 GHz 1066MHz 8MB Cache, Retail - £143.29
Scythe Miné Quiet Heatpipe CPU Cooler S478, S775, S754, S939, S940 compatible - £23.84
ANTEC MIDI CASE SONATA PLUS 550 SIL/BLK - £99.82
500GB Western Digital WD5000AACS Caviar GP, Green SATA II, 7200 rpm, 16MB Cache, 8.9 ms, NCQ - £55.21
Microsoft Windows Vista Home Premium SP1* 64Bit 1Pk OEM (DVD) - £61.79
Samsung SH-S203D/BEBN 20x DVD±R, 12x DVD±DL, DVD+RW x8/-RW x6, SATA,Black, OEM - £16.79
256MB HiS HD3450 FullHD PCI-E2, 800MHz GDDR2, 600MHz GPU 40Streams, D-Sub/DVI-I/HDTV Silent Heatsink - £32.91
Abit IP-35, Dark Raider, iP35 Express, S 775, PCI-E (x16), DDR2 533/667/800, SATA II, SATA RAID, ATX - £67.79
4GB (2x2GB) Corsair DDR2 6400 2x256Mx64 non-ECC 2x240 DIMM unbuffered 5-5-5-18 64bit OS DHX - £60.68
TOTAL: £562.12