As a follow up to my post a few days ago, I collect my car tomorrow, and am assured it is fully resolved.
The airlock in the cooling system was the cause of the poor heater function. Now that the airlock has been cleared, the heater is working as we would all expect. My agent has been road testing my car for several days trying to isolate another issue, so I know it's well tested. Fantastic service from Heritage Gloucester, including update calls each evening after each test drive home.
Feedback from the agent is that since identifying the issue on mine, they have identified six others with the same air lock. He even found an air lock on one car where the heater was working OK.
To test for the airlock, when you engine is at normal operating temperature, switch it off. Then feel the bottom hose to the radiator and see how it feels in comaprison to the top hose - by hand they both should feel the same. (Mine was completely cold). Additionally, feel the radiator at the bottom (again, mine was completely cold) and then see how far you can get up the radiator before it starts to get warm (mine was half way up). Obviously if you have a proper thermometer, that would be better.
Any of these symptoms, go see your agent .............
My interest was piqued with this air lock issue and so I resorted to YouTube and searched for coolant air locks on BMW engines. Sure enough, there are many videos detailing the inbuilt bleed procedure for cars equipped with electric water pumps. Using this procedure, bleeding is (apparently) simple. Not using it however, the engines are (apparently) very difficult to bleed. My supplying agent is a BMW dealer, so I would have hoped this would have been something they would have checked as part of the PDI process, but clearly not. Leaves me speechless to be honest, but just re-inforces why I switched to Heritage Gloucester - thanks again guys.