Windscreens leaking is an issue from the Range Rover Classic days!
My RRC leaked like a sieve, settled all the water onto the drivers footwell and rusted it to high heaven.
Is your leak top of screen or bottom?
I'd get a windscreen fitter to come to you and remove it, gives you the chance to inspect the frame at the same time and if it's not rusted out then maybe a reseal could fix it.
A few months back(last winter) my dad's Ford Exploder had a massive screen leak above.
It seemed that with a garden hose test, more water came in from the top of the screen, than my brother was blasting at the car! I got drenched, only expecting a trickle.
We took headlining off and I just blasted the entire inside of the screen with a tube of sealant from Sika. Can't remember which one to be exact tho. There was a small amount left in the tube and I took off the badly worn rubber seal from the top of the screen, and forced the rest of the sealant up into the gap between glass and roof.
Stopped it completely 
Looked as ugly as a finish could get tho .. but all he wanted on the hack was to drive it without a wet bum!
Arthur.
All these discos are giving me a heart attack!
'99 D1 300Tdi Auto ( now sold :( )
'03 D2 Td5 Auto
'03 D2a Td5 Auto
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