A few years ago I vaguely recall seeing somebody produce a hot water tank that fitted up into the ridge of the roof, hanging on the trusses.
Has anybody seen one? Am I misremembering? (wouldn't be the first time ;-)
IIRC the claimed benefits were that it freed up the airing cupboard space, and it would benefit from the rising heat in the loft (with a warm roof setup, not fibreglass on the floor).