I too had a bit of a dilemma regarding the do I or dont I luz ÂŁ5000 on a 2.5kWp system on a SSW roof....
I did and was amazed at the output and generation during the summer months and moreover the difference it made to the electricity consumption.
Im looking at about ÂŁ600/year in terms of FiT and money off my energy bills, but a lot depends on the way that you use the generated power for the latter.
Simple changes within the home, putting the wahing machine and dishwasher on only during daylight hours and not at the same time, the same with the electric cooker, switching to cooking more in a slow cooker (130w/hr) making sure that things are "off" rather than on "standby"
This graph shows this year vs last year energy consumption off the Norweb Federation, my PV system went live in July, though we had already made some lifestyle changes in the summer months which seem to be reflected in the June consumption data (LED lighting, smart switches etc).
Prices for PV installs do seem to be coming down, my friend paid ÂŁ9000 for a similar install 12 months before ours and of course is on the 40-odd pence FiT, we paid ÂŁ5000 an are on the 21pence FiT, now that the FiT has dropped again I expect the prices to have shrunk a little more.
Re: The Immersun, we have one and whilst this month has been disappointing (see "its dropped off" thread) it makes a blinding difference to hot water generation and I am confident that next summer I will be able to shut off the GCH completely...