The only problem with the cooling system since getting the one owner ZR in 2010 Was having no heat in the cabin, we had replaced the radiator, that was corroded along the bottom edge, probably due to a long term incorrect coolant mix, top and bottom radiator hoses fitted, then a correct fill of 5.5 litres of OAT coolant.  

Using a further trial and elimination process, the cabin heater control dial was set to hot position, radiator bottom hose detached and coolant drained off yet again. The first to check were the heater control valve, the cabin heater control dial, and then the heater matrix..

Cabin heater controlled coolant valve JJB-100240 attached to the bulkhead, is just a simple sliding coolant regulator or shut off valve, operated when rotating cabin heater dial, this had no issues. rotated a few times to confirm that the steel operating control rod was working correctly.

Next was a possibility of a blocked heater matrix, with a clean tube attached to the inlet pipe, a light blow through test expelled clean and clear coolant same with doing the outlet pipe, no blockage was evident. 

With heater dial still set to the 'hot' position, commenced on the filling procedure, till free flowing coolant emerged from coolant rail, the 8mm bleed bolt fitted back, then a check on coolant level, hoses etc, a fully working heater all year round, is essential.

Information is referenced only to our 1996 Rover 200 and 2004 ZR 120+