What geographic projection is used in GREAT-ER

[Johan Olanders]

Hi there,
I wonder what geographic projection is used in GREAT-ER.
I understand that I need to have the coordinates expressed as longitude and
latitude in decimal degrees, but what geographic projection is used. The
formula I'm planning to use for converting from UK national grid references
to longitude latitude needs information such as:

Fo; % Scale factor on the central meridian
a;   % Earth's semi-major axis in metres
b;   % Earth's semi-minor axis in metres

I guess that this information is available somewhere as soon as I know what
projection is used. I want "my" catchment to show up at the right place!

Thanks, Johan