My 2001 Disco V8 suddenly overheated whilst waiting at traffic lights today. After the 30 seconds the car returned to normal temperature and continues to drive normally!! Is this a warning on the head gasket? The car has a FSH and is constantly had oil checks etc.

This could be as you said that it may be the thermostat, or it may be the sending unit. I would take it to a shop soon before you do major damage.

chek if the electric fan is turning on when the ac is turned on

have the thermostat from the radiator to the block changed out.

sounds like fuel filter or pump

Overheating Due to corrosion block in Water Pump on Erratic engine coolant temperature or intermittent overheating can be caused by corrosion inside the water pump. ...Either condition reduces coolant circulation resulting in engine overheating.

Check for a air lock, could be a bad radiator.

hi your overheating problem could be the electric fan not working,you would not know this until you were stopped in traffic or were held up.Easy way to check is if your car is overheating whilst in town or traffic and not, when going along at 25mph plus,is check and listen to hear if fan is on when temperature is up at hot or over halfway.Could be the fan itself or a fuse.

You are correct.
There is an electric fan that activates via a temperature sensor. And this all is protected by a fuse.
Check the fuse panel it may be simply replacing the fuse will resolve the problem. If the fuse blows again have it checked for a short. Temperature sensor could also be bad and need replacing.
Hope this helps!

Need to put scanner or code reader on it ,to check for codes,before you can proceed. Not a guessing game.

it's tired!! needs a rest! LOL
check your fuel filter also check for vacumn leaks both are common problems on that model