300tdi cooling issue

Hi everyone. I have bought a complete 300tdi discovery with a very low mileage engine to transplant into my defender. It has been sat up for several years but runs like a sewing machine and starts on the button with no smoke. I have noticed though that the heater doesn't work unless you are working the engine hard and the header tank is stone cold. Im thinking it might be the thermostat stuck open but any other ideas would be appreciated. Thanks

Have you checked the thermostat and are you running the engine sitting on a pallet? These engines take a good while to heat up and sitting with it at idle you'll be waiting almost an hour before it heats up sufficiently. Take the thermostat out and test or replace, they're not expensive