This question is in parts:

1) My heater wouldn't pump out any heat, after jumping in the engine bay and physically moving the selector over on the heating box it workedm very well (serious heat)... until my fan stopped 5 minutes later.
Has the sudden heat f**Ked something up or is the fan switch just dead?
The switch has gone very soft (there is no resistance in the selector).

2) I was going to change the switch over for another similar variable resistor, does it need to be of a certain specification?

Thanks

have a look at the size of the fixed value resistor/s that's connected to the switch and you'll soon see you wont be able to replace it with the sort you'd normally see controlling the volume of yer ghetto blaster - better orf fixing the linkage