Nigel what is your opinion on 3/400 ft. of 32mm black pipe coiled flat on 8x4 ply base painted black enclosed on top with uv grade polycarbonate. placed on a frame on the ground facing the sun. The hot water from the coil thermosyphons up through insulated pipe to a heavly insulated 1000l tank on the roof with either a second coil or just direct feed from the tank back to the coil gaining heat during each cycle.  :)
A DC pump with panel could be used for circulation if the thermosyphon is not enough.