I might be talking carp here but if the water from my green hose pipe is anything to go by, would it not be feasible to just lay out a coil of about 100m of 32mm black irrigation pipe ?  Either on a flat roof or the deck (if you could fit it into the typically tiny "gardens" here)  :D
If one end was fed from the cold supply and the other to a typical butane water heater via a thermostatically controlled mixer valve, the heater should only cut in if it's input drops below 65C ?