Ok, thanks for the advice. I'm looking at the following now:
i5 2500k
8GB 1.5V DDR3 1600MHz (2x4GB) - Corsair/Crucial/G. Skill (whoever is cheapest on the day)
And then one of the following motherboards:
AsRock Extreme3 Gen3 (£105)
Asus P8Z68V-LX (£75)
AsRock Pro3 Gen3 (£85)
MSI Z68S-G43 (£75)
Looking at the specs I'm leaning towards the MSI at the moment. It's the cheapest but supports PCIe3, can run Crossfire (albeit x16/x4), supports USB3.0 and SATAIII.
What are your thoughts? In total it works out to around £270 with the MSI, which is pretty reasonable in my book.
Edit: Ideally I'd like something Gen3 and capable of crossfire/SLI support (just in case) but this is unlikely to get used in fairness, purely for the cost of getting such a setup, including a new PSU.