Deus_Ex: Have you thought about ordering parts form germany? Hardwareversand.de has some great offers and after looking around a bit they're usually a bit cheaper than UK shops for example. Here's a cheap build I managed quickly after looking for the best "value-for money" options:
Sapphire HD 5850 Extreme 1GB 106€ (a bit expensive, but FAST and cheap at this price)
Cooler Master Elite RC-330 38€
Super-Flower Amazon 80Plus 550W PSU 52€
AMD Phenom II X4 955 Box CPU 100€
OR
AMD Phenom II X2 555 BOX CPU 71€
ASUS M4A77T Motherboard 54€
4GB-Kit G-Skill Ripjaws 1333Mhz 39€
A total of 360€ with the 555 CPU, and 389€ with the 955 CPU (323GBP and 349GBP, respectively). The motherboard is great, the memory is low-voltage overclocking kind, the graphics card is a lot faster than a 550ti but with the same price tag, and the case you mentioned earlier (cooler master 330) is here also. What do you think?
That starts off looking like a good deal, but it's close in how much I gain by pushing it the extra money, plus there's delivery costs to take into account. Doesn't seem to add up to enough, on my tight budget. :/