Peter,
Did you resolve your heater issues?
There are a couple of photos in
this thread that I took many years ago of the heater linkages in situ. It appears that the heater control cable only acts on the hot water valve.
I have the heater box out of my (your old) 411 S5, having since fitted air conditioning. I can't find it at the moment, but I know I have it somewhere. Let me know if you still have questions re the heater and I will endeavour to find it.
Kevin