Have you checked the drains that go down the rear pillars, they could be
blocked causing the water to build up

Peter.

which kind of sunroof?

is it a tilt/slide or a tilting glass panel?

the latter type (as fitted to 300 TDi models) has drain pipes as mentioned.

I've solved a leak on the earlier type on mine, by adjusting the "hinges"
slightly.  Front one, in this case.

The seal should be the same as a peugeot 106.

Was your disco pointing slightly up hill?  If so you might suffer the same
problem i did.  As the drain tubes come off the sunroof they go up slighty
to match the contour off the roof.  Thus meaning the water tray of the
sunroof has to fill up quiet a bit before it starts draining the water out.
If the landy is pointing upi hill this as exaggerated if more to the point
where it will over flow from the sunblind attaches to the sunroof lower
tray.  I've found that with the sunblind mounting holes sealed up i dont get
any water in anymore ( even with the major downpoor we've just had! ) and
this is with a damaged seal too! Easy job once the roof lining has been
lowered

When its not raining try slowly pouring water into the lower tray of the
sunroof.  Look outside then at the rear of the landy for water,this will
prove the drain pipes before you remove the roof lining.  The rear sunroof
pipes are nearly impossible to rod out, so a airline will be helpful

MArk