Hi Andrew,
The Discovery has a very good cooling system so there is obviously something wrong. My guess is that the radiator is blocked and needs cleaning out,if it was the thermostat it would happen when your air con was off. The car is comming on 6 years old and if the previous owner didn't use corrosion inhibitor in the coolant the problems of a blocked radiator would be starting to happen. Cleaning the radiator out is not a huge job ,take it to a reputable radiator repair shop and get them to take off the top tank and rod out each of the cores(cost about$40US in Australia) then re-solder the top tank back on. Then get them to reverse flush the motor cooling sytem make sure you you a good coolant/inhibitor when you refill the system.
