Just to close out the saga....

1. Panel is really knocking out the hot water and still providing 100% of the DHW at the end of September. Compare to early July when it was struggling to get the water over 50 degrees C, even after days of wall to wall sunshine. Residual temperature at the top of the solar preheat tank from yesterday is still 56.2 degrees C

2. Any ideas what to do with a second hand panel which still works, albeit with reduced efficiency? Have advertised it locally for anyone that wants to have a go at something as an entertainment project, but not minded to try too much myself. One possibility that was thought of was to have it as a preheat for the cold feed to the DHW solar pre-heat cylinder. i.e. run the cold feed to the preheat solar cylinder through it in the summer time only (drain down in winter), but the 1 panel already does 100% of my DHW between March and September, so wouldn't bring anything new. Plus the pressure drop of running the cold feed through it, coupled with trying to remember to drain it down before winter just makes it a faff - Not to mention rigging it up..... It seems a shame to dispose of it though. I did debate a more intrusive post mortem examination, but it will be useless after that. From the before and after performance, its 100% conclusive its been selective coating failure.