how do you get to £65 per year?
If you very roughly assume that 1sqm of effective collection will collect something like 4kWh per day on average from May to September you will collect 4 x 30 x 5 = 600kWh per sq m.
Your alternative energy source may be gas (4p/kWh) Oil (7p/kWh) or electricity (12p/kWh) so each metre will save between £24 and £72 each year. A typical 20 x 58mm array of tubes has an effective collection surface area of about 3 sq m.
I dont know how many sqm of collection your £4,800 is buying, nor what kind of additional tank & coil and controllers the price includes. You can buy a 20 tube array for <£500.
However, as I said on the other thread, if the RHI rate for solar thermal is 19.2p/kWh then it suggest that DECC think that solar thermal payback is v v long. But that's for an MCS certified installation eligible for RHI. DIY'ing it though like globalti (who introduced me to the stuff) and I have done should pay it's way.
I have 9 sq m of collection which cost me £2k. SO I can collect more than the average 20kWh of energy I need daily for domestic hot water (9 x 4kWh = 36kWh). So if I were on Oil paying 7p/kWh for my alternative 20kWh then Im saving £200-250 year, or 8-10yrs payback.
However that's all moot for me as my alternative is "free" wood, but the solar installation means I dont have to lug logs around and light the furnace every couple of days throughout summer. So the solar thermal saves me time and effort instead.