Hey Pintman, thanks for the tip. Sorted now! I was a complete muppet. I'll blame VW anyway for changing the design, although it is a better design.
The wing mirror control knob on the GT TDI had 3 positions - L-Heater-R. The GTI has 4 positions - L-Off-R and Heater at 180 degrees. I couldn't see the heater position from my driving position so assumed that it only had L-Off-R.
Cheers.