Graham
Why are you running two brushless motors in a fast planing hull from a lead-acid battery? It will require a lot of power just to push the battery along and there's no chance that a SLA battery will be able to supply anything like the current those two motors will need to run efficiently. It's a bit like asking a forty-stone bloke with one arm to act as fireman on the Mallard. Lead-acid batteries have their place in a displacement hull which requires a lot of ballast and a slow-revving, low-current motor.
LiPo's are the obvious choice for the Fairmile because they can supply very large currents, but there's no reason why you shouldn't use NiMH batteries if the thought of LiPo's bothers you. Capacities are now up to 5AH for Sub-C cells.
Fairmiles should be fast and furious - not plodding and ponderous!
Dave M