Hi Colin
am not overly familiar with the 2001 Disco V8...is it the 4.0L "Thor" /Bosch engine? I recently did this job on my (late 1999) P38 RR. If so, the rocker cover nuts are those peculiar 12 pt head types and did not have washers...

The genuine R/C (neoprene?) gasket has brass tube inserts in the bolt holes which minimises the chance of over tightening (6 ft lbs only!) and oil seeping out around the threads. Dressing both sides of the gasket with a thin smear of loctite 518 esp. around the bolt holes makes for a permanent "fix"! No need for the o-rings.
Hope this helps
Hoges