Terry you said " I've just been in the roof and worked out why. The supply and return for the solar collectors are at the top of the tank, same level as the hot water outlet for the house. Accordingly, the solar heats the hottest water at the top, and then relies on convection and conductance to heat the rest of the water. My understanding is that it would do this inefficiently, as you get stratification of temperature levels in tanks. The thermosiphoning effect sets up only horizontal convective currents in the tank. However, when the solar heat gain is marginal, then theoretically I should get some hot water. In practice if it is cool and cloudy I get very little hot water."
Don't despair the solar panels do heat heat the whole tank. Behind those seemingly "too high" pipe inlets are specially designed vertical pipes which take cold water from the cold layer of water and then deliver hot water from the panels to the top of the tank. The reason the outlets are so high on the tank is to allow for a low height roof space while still leaving height for fall in the piping to the panels. Hope that explains it for you.