My Disco 3 has developed a problem with the rear door catch. When I unlock it and try to open the upper catch nothing happens. If I then give the rear handle a thump!! It opens!!

It locks OK.

Any thoughts/ideas anyone?

Lubricate, and/or adjust the latch fixings in the door. It could be under a little too much tension, not allowing it to unlatch. Try pressing inwards on the gate (press it closed kinda) and then try the handle again.

Or it could be a dodgy latch. They are very common failure on land rovers generally.