Hi,
This happened to my disco td5 (1998) as well.
I bought a gasket kit from ashcroft, put the disco up on ramps (front) in my garage, and took out the gearbox.
It takes a while to remove (a few evenings), because of all the siezed bolts on crossmembers etc, you cannot use an engine crane from the inside either. I used blocks of wood and a trolley jack underneath, and 2 people.
once the gearbox and transfer box are down you can take the gearbox away from the transfer box and then start replacing the gaskets. I found the only trouble was the gasket from the pump housing to the main gearbox, inside the bellhousing the bolts in this area were not tight, so the oil pressure just pushed the gasket out.
new gasket in and tighten the bolts properly, 10 minutes, putting back in 4 hours, no special tools.
A bit footery but not impossible to do yourself.
make sure if you want to do it yourself, that you have a decent manual to follow. (especially for mounting the torque converter.)
Mackies (ZF autobox specialist) in tollcross Glasgow gave me a quote for a full recondition, 3 days, over ?1000.....
That is why i did it myself... cos i is skint....
The car had been perfect ever since.
Cheers
Stephen