Thanks to everyone for all the help. I'm pleased to report that the heater is fully working again.
I ordered a new switch from Ford which cost £21 (for a tiny switch!). I fitted it and the heater worked again. It only worked on setting four until I realised the resistor wasn't plugged in
It now works on all settings with the original resistor. Not bad work in the dark at -3C.
So it was the switch all along! I would recommend people inspect the resistor before buying a new one.
The resistor is a nightmare to get at! I had to make this monstrosity to get at it:
The red tape on the bendy joint is to provide some rigidity!