Join Date: Jun 2015
Location: Dalgety Bay, Fife
Car: Mini Cooper, MG TF, MG Midget
If the heater is either hot, hot or hot no matter where the knob is turned it suggests the water valve behind the centre console is permanently open no matter what position the knob is in. Which also suggests the cable has become detached from the valve and is not moving it, or the valve itself has gone "open circuit" internally.
I don't think it could affect your leak, as such, unless the valve is really faulty and is leaking as well as not working as a valve. If it is leaking that much your carpets would be very wet, I would think?