And
here is the correct method of refilling/bleeding a 75/ZT diesel Josh.
To increase your chances of success at the first attempt, I would follow MG Rover's recommendation that the system is first fully drained (after which you can back-flush the heater matrix as
Trikey suggests)
including the cylinder block drain plug.
I don't have a diesel, I have a petrol 75, but by following the MGR procedure for my engine exactly,
I have never had an air lock, overheating or poor heater performance.
Let us know how you get on.
Simon