photo for a magazine

i have been asked to take some shots of a olympic hopefull horse rider for a sports magazine, i have only ever done things like portraits and weddings and i have no idea how much to charge for this i live in the uk so any help in £ pounds would be great.
they are going to use the pic for a full page spread
does anyone have any idea on this kind of thing?

thanks for all your help

Oscar,

I have been a magazine photographer for many years.
It all depends on the magazines circulation and budget.
I have sold pictures for as little as $50.00 and I have sold
pictures for $2000.00

Ralph