For most outdoor appliances a double pole switch would be my first thought.
I have recently installed a timer switch, and it has very affective and low cost.
Let me get the model
Elkay also do some timers too, never had an issue with any of their products
If the double pole doesn't fix the problem is there anything I could do regarding the trip i e change or fit some suppression etc.
Whats your cable run
You could look to change the RCBO for a 40mA trip, but that would be my last resort
About 20 metre from switch to motor
So your not talk long then
I often refer most customers to the different artciles found here, this one is probably as comprehensive as you will find.
A thermo-magnetic breaker might be an option
All that said I am confident changing the switch will fix your issues.
In terms of supression, you dont really want to open the casing of the motor if its IP66 rated and its the motor you need to get to, in order to surpress back EMF
If I were you pick up a new switch from Screw Fix, they are often boxed swap the terminal connections over and test it, if it gives you the same fault return it back to Screw Fix.
I would appreciate if you could rate my response \ service today, should you need any further clarification please let me know.