It's not far off a 6000 mile service, so first off has it been serviced in the past to schedule this will determin what has to be done. Most owners doing a diy service do everything in one hit.
6000 mile or annual service is the basic one and is just an engine oil and filter change and with a general visual check, but if your disco is used off road then oil in the gearbox, transfer box of diffs may also be changed.
Below is a link to a servicing schedule for the disco 1 you will also find a schedule on the www for the D2.