My Land Rover was overheating , so I replaced thermostat its still over heating?

It MAY be, but a good reputable mechanic will do a compression test before tearing the engine apart. The head and/or head gasket can fail between the water jacket and the combustion chamber and still not have water in the oil. That would definitely cause the overheating problem, as well as the smoke.

A new head gasket is a common problem with the discos. But before you go that route, it could just be oil leaking from the valve covers. I have seen this many times. You will need a 16 point socket and just tighten the valve cover bolts. They will be loose as the dealer spec is something like 4 pounds of foot pressure.