Hi and

Sorry to say you may have inherited somebody else's problem and you may want to get your money back now as this may get expensive but if your happy to have a go then read on.
First thing to check is the water pump as the impeller can become detached or even break up.
You can check the pump by removing the bottom hose housing from the block this is also where the thermostat is.
Make sure the impeller is fixed to the pump shaft, If that looks ok refit.
You also have an electric water pump on the bulkhead behind the fuel filter this should run all the time engines running and for 3 minutes after switching off .
Also check return pipe to coolant bottle is returning coolant while engine is running, you should see a small quantity of coolant returning through the small hose at the top of bottle .
Try running the engine with the cap removed and observe the coolant bottle does the level stay steady or does it rise and flow over continually.
Combustion gases leaking into the waterways causes continuous air locking, pressurising, overheating, fluctuating coolant level, cold heater.
All of the above can be caused by head gasket failure.