Symptoms:
Speed control is not working, it would go from Stop to Full Power once turned on.
Mark the cable position so that you do not forget, and unplug.
Remove clips on the white cover and gain access to the electronic module. It is quite simple and the likely issue is the TRIAC. Order and replace this.
The TRIAC is BTA208-600B. I bought mine from x-on.com.au. They are only $1 each and I bought 3 of them as spare, shipped for $20 to Melbourne.
Link: Triac BTA208-600B
Once replaced, the speed control works properly :)