It sounds as if the circulating water is stone cold - which probably means that the thermostat is faulty, ie jammed open. It's the first thing to check anyway, and a replacement is cheap.
If you find the thermostat has been removed by someone, then you'll need to find out why - it might have been overheating and removing the thermostat before selling the car hides the problem. This would need to be investigated, but rather than anticipate complex problems, the first thing to do is just remove the thermostat & have a look at it.
Land Rover Defenders have fairly basic engines and spares are quite cheap, so don't panic. It might be worth finding a self employed mechanic to help you if you don't have confidence - there's very few tasks that need a main dealer to be involved.
Good luck,
John