My parents had 2 panels fitted about a year ago. They live in Mortimer West End so a similar climate to Tadley(!) actually, next to the graveyard too but I digress.
My Dad and I both being geeks, we built a system to measure the energy delivered into the hot water cylinder. (Measure temperature difference across the heating coil and combined with the flow rate you know what's going on).
I will check on the latest stats but IIRC it delivers between 3 and 5 KWh on a bright summer's day. It rarely generates enough heat that you can rely on it for 100% of the hot water, purely because it can't heat the whole tank hot enough. Possibly, it is trying to heat too large a tank but once the tank is around 50 degrees the heat input rate drops off because there is not enough temperature difference between panel and tank.
They certainly contribute (and my parents have a choice of leccy or oil heating so every little helps) but they will have shuffled off this mortal coil long before it pays for itself, I suspect.
Certainly the marketing claims that these panels provide anything useful "even on dull days or during the winter" are rubbish. They need bright sunlight before they start to work. They do start working very rapidly once they have sun, though. They do their best work between 5 and 8 in the morning in the summer, after dawn and before the cloud sets in for the day.

Kevin