Not a hard job and should take around 30 minutes.
It maybe handy to get a new rocker cover gasket though and the part numbers for the gasket and caps are:
LJC100270 - Cap-valve stem
ERR2409 - Gasket-engine rocker cover
One tip when it comes to checking the valve clearances:
Chock the rear wheels, jack the drivers side front wheel just clear of the ground, so you can rotate it.
Put the Disco into 2nd gear and now you can turn the engine over just by turning the wheel in the normal forwards direction. :D
It just saves trying to do it with a socket on the crankshaft pulley as there ain't much room down there.