Well that changes everything!
if the US are willing to GUARANTEE the cost will only be £ 400 miilion its an easy decision (tho the rip-off merchants building the crarriers will have to find another way of charging squillions for their bottom line)
if the US are saying their best estimate is £ 400mm then I think we'd look at their own record of forecasting costs on the B-2, F-22 and F-35.....................