Seat Alhambra - rear tailgate light water leak?

We've just bought a Seat Alhambra 2.0 TDi 58 plate and there is a leak in the tailgate lid. When it rains it lets water into the inside of the tailgate trim and when you lift it up to open the boot, it leaks out wherever it can. Has anyone had this problem also and if so, how easy is it to repair yourself, or did you take it to a garage and how much did it cost.