Last night my Sparky friend came around and this is what we came up with.
Basically the switch on the left needs to be on in order for the exhaust fan and controller to beon - anything ever goes wrong with either and I can isolate the power completly.
Then the switch next the the controller - off = Auto (the controller controls the exhaust fan); on = exhaust fan always on (like an override).
For the controller
Basically it was a TH20 Hunidty/Tempature controller from ebay. Cost about <$50 from memory.
You can set the Temp (Alarm 1) and Humidty (alarm 2) seperately. Each "alarm" state closes a seperate relay.
I am only using the humidity, still trying to find the spot I want.
I have started and when it hits 65% RH, active the fan until it drops by 5% to 60%.
I let it run like this for most of the day and then went in and "checked" how the room felt - a little to siticky for my liking. I have pulled it back to 55% as an on point down to 50% - will check it out tomorrow after another full day.
You can program it to either active the relay and drive the value "Up" or "Down" in increments of 0.1% for a difference of x
For example when the pic was taken, I have set Alarm 2 (AL2 in the above - not active) to switch on at 60% for 5.0 difference. Works great.