Following a slightly leading question from my colleague David, I've done some
quantitative analysis on the potential savings now and under a hypothetical Renewable Heat Incentive regime.  The RHI will now require metered amounts for which I've used the modelled kWh savings from a solar  thermal system over a non solar thermal system.  In reality these may be below the actual meter amounts as these will include any storage losses.

.

I've modelled:
   -  1) an A rated combi boiler  (although the house and number of bathrooms is too large for a combi)
    - 2) an A rated combi boiler with low flow showerhead
    - 3) an A rated boiler with a solar ready 250litre cylinder
    - 4) an A rated boiler with a solar ready 250litre cylinder and ultra low flow showerheads
    - 5) an A rated boiler, solar thermal system and 250litre cylinder
    - 6) an A rated boiler, solar thermal system and 250litre cylinder and RHI payments
    - 7) an A rated boiler, solar thermal system and 250litre cylinder and low flow showerhead
    - 8) an A rated boiler, solar thermal system and 250litre cylinder and low flow showerhead and RHI payments

Not done yet....I've also looked at the paybacks with fuel prices and RHI prices remaining static as well as with a 10% annual increase in fuel prices and a 2.5% increase in RHI payments.

I've applied costs that are either marginal (i.e. for solar thermal) or that apportioned to the hot water (i.e. in the case of a boiler).

And here are the results:
       Annual Saving        Install Cost             Payback                        Payback                        20 year savings
            (year 1)                                        (static prices)        (with prices increases)            (less capital outlay)
1)          £125                    £200                    <2 years                        <2 years                              £6,959
2)          £143                    £250                    <2 years                        <2 years                              £7,940
3)          £107                  £1,000                   <9 years                        <7 years                              £5,071
4)          £146                  £1,050                   <8 years                        <6 years                              £6,853    
5)          £171                  £3,500                  <20 years                      <12 years                             £6,638
6)          £309                  £3,500                  <11 years                        <9 years                            £10,163
7)          £195                  £3,550                  <18 years                      <11 years                             £7,676
8)          £310                  £3,550                  <11 years                        <9 years                            £10,614