Well after faffing around for 24hrs or so I've ordered a system in bits from Lambdatek.
I decided to go for an Intel G45 based system which might be a little bit "too new" for safety but what the heck.
It's an Asus P5Q-VM motherboard, core 2 quad processor, 8Gb RAM, 500Gb disk drive, £473 (incuding a few more bits I haven't listed).
I *nearly* ordered from efficientpc.co.uk but in the end the above system just about beat them to it.