I has a similar issue. Found a work-around. Try this befoe going to any expense.
1, turn fan dial to full
2, turn heater dial to full till is engages air con (yellow light next to dial)
3, Only at that point press the heated windscreen
I can only say it worked for me. If I try any other sequence then nothing on the drivers side.
Zetec Ecoboost 1.0 100ps Deep Impact Blue