Insolation in the uk

[lurksalot]

I am doing a few calcs regarding heat gain through windows and I am looking fot some typical values to apply for vertical surface NSE&W.
I currently have figures from a glass manufacturer, a net based calculator and the BRE standar assesment proceedure ( for energy rating houses).
All three are wildly different with the net based one looking most likely , but does anyone know of any standards or models that can provide a reliable figure I can use.
many thanks in advance
Kev

 

[lurksalot]

I am leaning further to the net based calculator now.
But does anyone have any knowledge of gain figures or any idea where to find them

Kev