For all you guys who helped get me educated, you might like to know that all is now well and I can at last agree that solar water heating is the great idea I had thought it was going to be. It got worse before it got better. The manifold indeed packed in and started leaking and two tubes filled with water. I had the case all ready to go to tribunal but was held up by the difficulty of getting an independent report on the installation. The local master plumbers assoc. brought up the case at a meeting and Stephen Brooke Taylor took over and managed some sort of miracle which saw the installer finally agreeing to take responsibility for his installation and have it sorted at his expense. The whole thing was replumbed without the 5 way valve. The manifold was replaced complete with a proper steam vent. The header pipe was extended (great showers! I'd forgotten how nice they can be) and the controller replaced with a sunshinesolar version which has a Holiday mode which can cool the system overnight. There hasn't had an opportunity yet to show me what it will do when it stagnates. Fingers crossed.

BTW you might be interested to know what was blocking the 5 way valve. It came out with a thing like a big grey slug attached to it, This turned out to be a sheath made of some rubbery compound which apparently is supposed to insulate the hot flow pipe from the cooler water going in the other direction as it passes through the HWC. Obviously not intended to cope with super heated steam, because it had melted and got sucked into the outlet from the HWC. Which explains all the subsequent behaviour.

Thank you all for your forbearance and your support. It has been much appreciated. 

Linkback: https://forum.plumbers.nz/solar-heating-and-heat-pumps/9/happy-ending-to-sad-saga-re-coryluss-solar-water-heater/1566/