Has anybody come across a problem with the HOT/COLD control knob on the Mk1s with the 4 speed heater fan where from HOT its blowing barely warm air yet if you slowly turn the knob (say leave it just where the HOT markings finish) anti-clockwise towards the COLD you can feel the air getting slightly warmer?

Not sure if the knob control is merely connected to a cable and flaps thus a mechanical or whether that knob controls the heat electrically and maybe requires renewal?

Any ideas. My feet seem to freeze. New rad, matrix, pump, thermostat, car gets to temp quick and reads normal, 50/50 collant mix. no leaks. no airlocks.

Sounds as though you may have an air lock in the system. If the heater matrix was replaced it may need to be bled again.