Nigel, if you want to drop me your contact details I'll happily undertake a bit more research on suitability for a 5kW Iskra for you. The performance of this model is broadly similar to the 6kW Proven (see output graph for Hockerton site which has one of each). Payback is tricky - since it depends on your view of future energy prices for your imported energy. Linear price increases in line with RPI+ or inflation are clearly nonsense.

I can look at pricing for supply and install for you too, and provide reference schemes if necessary. No obligation, happy to help and provide advice.

Regards
Colin
brownecow@hotmail.co.uk