the original valve seals break down and they leak both internally allowing hot water past and externally through the spindle, hence the coolant smell.
get yourself green and my replacement valves are available from the shop. Ic cold air is an option with the new valve fitted and good aircon.
I'm in! It's indiana Jones finding a lost cave but without the snakes, tip top.
Do you think from my description that the heater valve is defo on the way out?
Obviously I will look for the tell tale signs tomorrow as phil advised.