TomGeorge:
When you phase control, you control when in the AC 1/2 cycle the triac conducts'
The earlier in a 1/2 cycle the longer time the load is connected to the AC, so more energy hence with a motor more speed.
The later in a 1/2 cycle the shorter time the load is connected to the AC, so less energy hence with a motor less speed.
If you use a zero crossing type, it will not control where in the 1/2 cycle the triac turns on, it will try to get the triac to ALWAYS turn ON at the start of the 1/2 cycle.
This and many other YouTube videos explains your controller.https://www.youtube.com/watch?v=4N1uLth1o9o
Tom...
thanks for that video that helped a lot, i think you have solved the problem, its almost certainly this opto, hopefully the new one will arrive soon and we will see if it fixes it. thanks for your help!