jakeclyro is right and hoodoo is wrong about paybacks. FITs turns the old financial model (hoodoo) on its head.
molgrips, I think the OP is interested in saving energy, that's why I suggested it. OP said, "I like to think I am as Enviromental as possible", which I took to mean saving energy and reducing carbon emissions.
Bear, air source heat pumps (ASHP) and solar systems actually make a good match. (it's not like the ASHP gets in the way of the panel, one is generally at ground level, the other is usually on the roof). With solar hot water, you can reduce the heat demand from the building and therefore the work the heat pump has to do. With PV you can provide some of the energy to run the pump. Neither of these however, addresses the fundamental problem with ASHP which is poor preformance in winter, when the external evaporator coils may freeze up. All a bit irrelevant as ASHP
is not a good choice for any property on mains gas.
matt_outandabout, dropoff and jakeclyro are talking sense.