I would generally concur with most posts here, in that:-

1. It shouldn't be JUST about payback periods for installing solar panels (and any other green technology), its each person making a long-term commitment to reduce the dependancy on fossil fuels as an energy source - a big THUMBS UP for those who have done so!
2. For those of us who need to look after the pennies at the moment, it's generally accepted that using solar panels to heat hot water is a much better bet at present, as the technology is more mature, does not rely (so much) on government subsidies (which ironically may be withdrawn if schemes become too popular [a typically short-sighted attitude from the previous UK government, let's hope the current one is better]) to be "viable" from a payback point-of-view (can be well over 15-20 years in many cases for photvoltaics, as opposed to 3-8 years for solar hot***er).
3. Assuming you have access to a south-facing pitched roof, most solar hot***er systems should provide about 30-40% of a house's annual hot***er demand, which is not bad going.

When I specify such systems when working on new-build housing projects, they (solar hot***er systems) make a very effective contribution to reducing properties' carbon footprint (better than photvoltaics), and can be a very effective selling point given the relatively rapid payback period.

One word of warning, though - as with all "green" technolgies, there are many companies enetering the market now, several of which have no experience in such technology (they just re-sell others equipment, rather than make their own), some of whom are decidedly dodgy customers (I suspect some may be featured on BBC Watchdog in the coming years), so watch out! I would go with:

a) Recommendations from friends and family as regards the service (not an appraisal of the technology) provided;

b) Look for established firms with a track record of providing quality equipment and service - many boiler manufacturers are major players. Some other firms providing many green energy systems will also have a good deal of experience. Make sure that your needs and options are explained fully. in simple terms that you can understand and without the hard sell - a sign of a reputable firm is one that tells you NOT to buy their most expensive kit available, and is open to showing you other installations (go and see them if you can - a "test drive");

c) DO NOT pick the first package you fell meets your needs! Take your time before you choose, and also evaluate alternatives (it may be more cost-effective to replace your crumby old 65% effecient boiler with a 90% efficient one first, or to have cavity-wall/roofspace insulation/decent double-galzed windows installed first).

d) Make sure you aren't wasting energy needlessly around the home (one of the Greater London Authority's [GLA] good strategies is "Mean, Lean, Green")

• Mean - use as little energy as possible (buy energy efficient equipment, don't leave them on/onstandy, leave windows open in winter unnecessarily, etc);
• Lean - where you do need to produce energy (e.g. heat for hot water/heating your home), do so as efficiently as you can (make sure the boiler [for example] is a reliable, easy-to-service/use and long-lasting unit);
• Green - Use equipment/primary energy sources (fuel) that are the least environmentally-detrimental as you can afford (see payback above) whilst meeting your other requirements - note that this should include transportation of the fuel (wood pellets sourced from over 30 miles away for boilers is not so green), servicing (not the financial costs - how far/often the service contractor needs to visit, fitting/transporting/making replacement parts, longevity of equipment). A good firm should be able to provide you with such information. A local, reputable firm may be better in may cases than a national one.

Sorry for the long post - I seemed to be "in the groove" for writing it. I hope it's helpful.

Just had payment of £460 for three months feed in tariff.

You wouldn't get 1/4 of that if you were paid the market rate for intermittent electricity. The trouble with solar is rather like wind-it's either sunny or windy all over north west Europe or nowhere-so either everyone wants to pump their glut of low grade electricity into the grid and pick up their 'green' subsidy or they all want to tap into the grid and leach off the baseload. That's the problem with most of these 'renewables'-they usually end up simply displacing carbon emissions. In this case it is operating the carbon intensive baseload at low load ready reducing the efficiency.