Bob Mannix typed:

Could you furnish any diagrams on your system? The commercial one
referred to did have the advantage of being (almost) entirely
non-invasive to the existing system.

Well ive got a few things going in my favour, I live on the south coast and
in a very sheltered valley the house having had many alterations over the
years has a roof ideally suited being a u shaped /\_/\
Theirs nothing special about the design, it's the same as the conventional
example shown on http://www.solartwin.com/ but all the parts were obtained
from boot sales or farm auctions, Its got 4x 6ft radiators in a wooden frame
with Perspex covers sited in the V of the roof facing S-SW.
Pump is 12v and powered by elec solar panel 1x1m, the only mod I had to do
was the pump motor stalled when Volts dropped below 5v,
(which also meant that there wasn't sufficient heat/light either)
fixed with 2 Zener's and relay.
We use a LOT of hot water during the summer (three B+B rooms) and hot water
is by oil boiler so every 1degree saved is worth while IF the initial outlay
is modest.
Winter I don't care as we have a HUGE wood burning stove.
HTH

--
Mark