From http://www.lowenergyhouse.com/eco-home.html:

The roof is orientated south at 36 degrees from the horizontal to   allow maximum solar gain for the solar collectors positioned on it.

From http://www.feedintariffs.co.uk/solar-pv.html:

Technology Type	Scale of Technology	01/04/11	Tariff (Years)
Solar PV	0 to 4 kW (New-Build)	37.8p	25
Solar PV	0 to 4 kW (Retro-fit)	43.3p	25
Solar PV	4 kW to 10 kW	37.8p	25
Solar PV	10 kW to 100 kW	32.9p	25
Solar PV	100 kW to 5 MW	30.7p	25
Solar PV	Stand Alone System	30.7p	25