Since the OP is paying for what he wants in GBP then there are a few things to note here in the UK.
Currently 6990 manufacturers are pricing these to the point that they are cheaper than 6970CF. So in terms of current value, you're not losing out if you opt for a single PCB that has multi GPUs. In games where the 6970CF beats the 6990, the numbers are academic at best.
For almost £100 less, anyone opting for a multi PCB setup should consider the 6950CF. For current games atleast and at stock speeds they perform almost identical to the 6990 in some benchmarks and only losing out when the 6990 is OC'ed.