A very common problem on Discoveries is the gearchange getting difficult. It's most noticeable when selecting reverse but can also be felt when changing to first and sometimes second gear.
This may have be caused by low oil in the clutch master cylinder, so check this first. But if oil is fine chances are good the problem lies with the so-called "clutch damper", a little device I suppose should take out the shuddering when engaging a gear. This little item looks like a thick disk, about 2 inches in diameter and costs around 20£. It sits very low in the engine bay and can best be accessed from the right side under the car. (picture right looking up)
The problem is loosing the flexible rubber line. I disconnected the metal line at the clutch slave cylinder at it was frozen in the damper side, than loosened the flexible line and removed the whole thing by loosening the bolts that hold the bracket and the slave cylinder together in place. Take great care not to withdraw the slave cylinder from it's position.
Once removed you can disconnect the flexible line, than remove the 2 bolts holding the damper in place. Discard the damper, it's not serviceable. Remove bleed screw on slave cylinder and clean it.
Fitting the new is exactly the same. Take care to mount the flexible line first. Then screw in metal line a bit, mount it all in place and tighten up.
Bleeding is a different can of worms. As the brake line runs higher than the master cylinder you will have trouble getting the air out. I know of some who used an old seringue to push oil up from the slave cylinder bleed screw. This works. Another method is to keep on pumping until it works. This may take several dozens of movements but once oil starts to move you're a winner. This is best done with a friend who watches the reservoir and fills it up continually.
As it's not that easy to get the air completely out you may have to drive some time until all the small air bubbles will have moved up. This sounds silly but it worked for me. After almost no change the situation got better over a week and now it's fine.