OK - given the wet summer we've had, some might think we were being slightly optimistic, but the clever design of the panels as tubes gives an increased surface area to make the most of any available UV radiation, meaning that even on overcast days there's some degree of water heating.
We also had the hot water cylinder upgraded to a massive 300 litre capacity.
Initial verdict - well we've had a couple of weeks of September sunshine and the 2 panels have managed to heat the cylinder to an impressive 65C over a day. So we're happy with it.
Tidy little panels as well...