We use an 8 amp float switch available from machine mart. They are designed for submersible pumps but have the feature of being able to work the other way round. ie switch off when full rather than switch on when full when used with a submersible pump.
It is secured from the top if the IBC tank with a piece of conduit about 200mm in length. We heated the bottom of the conduit and folded it out over on itself to provide a radius for the cable to pivot on. (The reason why we put in this length of conduit is that we wanted to empty a third of the tank before the r/o kicks in. An r/o works more efficiently once it gets going, so this was a way of keeping it filtering for longer periods and save on resin.)
We have wired this into a 230v normally off solenoid valve which switches the water off to the ro once the tank is full. As this float switch is rated for 8 amps, it will also run most booster pumps. We don't run a booster pump though. The Clarke pump is 1000 watt which is a little less than 4.5amps. This is within the float switches capacity, even when considering a little more current required on startup.
So once the float switch activates the solenoid valve to switch the water flow on for the r/o it will also switch on the booster pump. Once the float valve switches the water off when the tanks full, it will also switch the pump off at the same time.
We also have a overflow pipe fitted onto the side of the IBC tank with some hose to a drain incase the solenoid valve fails. We have had this setup working for about 5 years now with no maintenance needed.
As is the case with all installations, 230v and water aren't good bedfellows. It is essential that any electrical installations use an RCD to protect against shock should any electrical problem occur.