Well actually, DarkMX1000 got back to me and said 6870 as well. He said if I can get my CPU to 4GHz I should go 6870 CF.
He hasn't replied back again (v. busy), but what about 6850 CF? 3.8GHz?
Essentially it's 6870 = £155-£165. 6850 CF = £250. 6950 = £195. I wouldn't unlock some of the shaders on the 6950 by flashing it to a 6970, not only could it be unstable ('binned chips', and less power going to the card - there's a reason for that) but I'd want to keep the warranty.
So a 6950 is a bit faster than the 6870 but a vast amount slower than 6850s in CF, which I'd be wasting power on.
Crank the CPU to 3.65 or 3.7GHz and go 6950?