I've taken the temperature of the water in the tank, purely solar-heated.
A couple of days ago it was 29°C. This morning, after a much cloudier afternoon, 23°C. It's quite annoying, actually, as when the panel was fitted in 2005, I called the guy back because in the height of summer, our chimneys cast a shadow of the PV panel which drives the pump, and we were missing out on several hours' hot water a day. He shifted it to the bottom RH corner of the hot water panel and now, from mid-November onwards, next door's chimney casts a shadow over it in the mid-afternoon - the point at which the pane gets the best of the sun. I suspect that reduces the temperature of the water in the tank by a few degrees when it happens, but usually there are so few days in a year in which it makes any difference that it's not worth sorting out. The call out charge would mean that it would take a few centuries to pay for itself.