ebay solved my problem
Managed to get an entire 2nd hand heater control panel for a much better price than the single switch quotes I had earlier. With a bit of luck once it arrives I'll have my problem solved.

Wondering about why the resistor pack thermal fuse blew in the first place though - this was the first thing I tested and on finding it had blown I replaced it with a new one from Maplin.

Now I'm wondering if, once I've fixed the switch (also clearly faulty), the original problem will rear it's head again and my Maplin thermal fuse will blow. Having checked the blower motor already and found it to be working as far as I can see, I'm thinking there must have been another more subtle problem originally to make the thermal fuse blow. Or would it have done this purely due to old age?