.."Bob Mannix" wrote in message
...

Having revisited the scene I find kits are now available eg:

http://www.solartwin.com/

Just taking this as an example, a DIY kit, with a simple plumbing
arrangement (for those of us with tanks!), with an additional pv panel to
drive the pump, is £1900.

I would surmise savings in the region £100-200/year (guess) giving a payback
time of 10+years.

http://www.solartwin.com/payback_calculator.htm
suggests about 1000 kWhr a year, which is reasonable.
They only suggest £100 a year saving in exceptional circumstances.