Head gasket problems causing water loss on the M series are very rare most like causes are the rad or water pump or one of the metal pipes. Rover crossflow rads get pretty tired around the 100 k mile mark partly due to metal fatigue caused by resonant vibration at certain combinations of rpm and road speed. Also the pressure vent cap on the header tank is prone to sticking. Best advice is to get the system presure tested to about 15+ psi when COLD --- testing when hot is no good as tiny leaks just flash off as saturated steam which can't be seen .
If doing any work on the cooling system on these engines I advise renewing any iffy looking hoses or pipes -- the very short joining hoses are often over looked when these blow on some 2 litre models they can damage the engines ECU.