Thanks for the recommendation but I have decided to get a single 7900GT for now. I have obtained one for around £220 including delivery and VAT. It seems they overclock extremely well and should perform not far below this ATI which is £100 more. If I want to upgrade further I shall get an additional 7900GT which will blitz the ATI model you have shown me.
If I had a Crossfire motherboard I might lean more towards the ATI cards, but the 7900GT seems to represent the best VFM to me atm, and I have an SLI mobo.
I'm still open to other opinions, and I have at least a week to change my mind whilst my supplier gets new stock.