Finally after three days of shopping around I've ordered this system from RL Supplies:
AsRock P55-PRO
Intel Core i5-750
GeForce GTX 260 (896MB)
2MB 1333MHz DDR3 (I can add more RAM later)
Maxtor 750GB SATA2 HDD (7,200rpm, 32MB cache)
Antec EarthWatts 650W PSU (Antec are good, right?)
Arctic Cooling 120mm Low Noise Case Fan
Arctic Cooling 80x25mm Low Noise Case Fan
Comes to £600 with VAT. No delivery charge because I'm collecting from the store on Wednesday.