E39 V8 - cabin not heating on side

After a brief drive today (very cold day) my wife has reported that the drivers side ventilation is not giving out any heat.

At first I thought that it must be a control valve thing but then I remembered that the expansion tank was leaking slightly at the bottom and that the coolant could be very low. From my understanding of the cooling system I think there are two water inlets to the heating system from the engine. Could it be that one bank of the engine had run low on water hence no heating?

I have since removed the expansion tank and level sensor to find that it had completely destroyed itself internally. The tank is full of broken debris and must now be replaced. The sensor was leaking water as fast as I poured it in and was in pieces too - hence no 'check water coolant level' warning.

My fear is that I've cooked at least one bank of cylinders although the temp gauge has never moved and the engine hasn't done any long journeys.

I am waiting for the new parts to arrive - until which I have to wait and see

If anyone has had something similar please reassure me that my engine isn't fried.

If nothing else - get your own expansion tank checked or replaced. £50 all in for the parts and 30 mins labour - it's really easy to do