Finally a thread on my area of expertise! I've worked in green construction for around 10-12 years as a QS and I've specialised in solar for the last 3-4. I have my own company too so if you want some impartial advice please just drop me a line. We don't do hard sales tactics.
To respond to a few points already raised:
we have a 16 panel system which is 4kw the maximum allowed on a private dwelling
You can actually install as many as you like, though 4kWp tends to be popular as it's a good return on investment at that point. My company are commissioning a 120kWp domestic system this week (though that's quite a special house!)
I was a bit concerned about the extra load on the roof, but I guess they factor that in.
Always worth double checking the installer pays a structural engineer to sign it off. Plenty don't - it costs around £100 so we insist on it on every project.
I do think it is unfair that only properties with a south facing roof reap the most benefits of solar.
Depending on your roof you can still make very good money on non-South facing roofs. A recent NW facing roof we installed will earn 13.63% - that's got to be ten times what most bank accounts earn these days and unlike most savings accounts that money is tax free.
At 17p per kwh (thats one tarif for here, and most likely NOT what the grid pay you), you have made £670/year with them, which sounds great, but would take, 18 years to pay back the 12,500
If you installed today you'd get 14.9p Feed-in-Tariff, 4.64p for electricity you sell to the grid and you'll saving around 12p for every unit you no longer buy in. We assume 50/50 usage / sale to grid - based on that you get 14.9 + (0.5 x 4.64) + (0.5 x 12) = 23.22p per unit. In the case above that gives an income of £915 per annum (all money being tax free). Also there's no way a 4kWp will cost anywhere near £12,500 today - the current cost is nearer £6,000. That gives a return of around 15% and a payback of 6.6 years based upon the generation figures One eye mentioned.
I'm not sure when the feed in tariff was changing again, as it keeps reducing.
It's reviewed every quarter - it's due to change at the end of March though the drops are minimal now - on domestic (sub 4 kWp) it goes from 14.9p to 14.38p.
Even the big companies claim 6-8% return. So if you invest £12,500, you can expect to make only £750-£1000 in 25 years.
6-8% sounds quite low, plus that's the rate PER YEAR. So you'll be paid £750-1,000 every year for twenty years (plus the figure goes up in line with inflation).