The Heater resistor pack, the green think under the dashboard, has a thermal cutout, if this is open circuit the 3 reduced speeds will not work, position 4 (?) will still work.
The thermal sensor is spotwelded in , because it gets hot in there, looks like a silver diode. If tested with a multimeter it should read 0 Ohms, if it reads high Ohms or O/L its buggered. They are in the RS catalogue, i just got one to replace mine, 172 Deg C cutout, 10Amp. Part No. 176 9312. The trick is to crimp the new device in on to the old leads without pullling the spotweld off