heating water using the sun Answered

Hi
I want to install a hot water system, as cheap and simple as possible.  The best idea I have found is to connect a new pipe to the main storage tank, and run it the 70 metres to the house, overground, so the sun heats it.  This will work, but it needs a one-way valve at the tank end to stop the hot water entering the storage tank and heating that.  The normal plumbing valves for this need a high working pressure, but our supply has only 2 metres of head at the point of connection.  Anyone know how I can get round this?
Phil