I don't understand the figures here. If one assumes that you can sell you electricity for 7pence and get an additional 3 pence for the ROCs then you can earn an average of 10pence per Kwh. To earn the expected payback of £450 would mean production of 4500 Kwhs a year. This is a very debatable claim. As there are only 8760 (24*365) hours in a year, this seems to assume the turbine would be operating at peak output half the time. This is nonsense.
I support small-scale machines and wish more people would have them, but don't want people to feel let down by unrealistic claims of their "rule of thumb output"