Although it's a long way from being the stock answer, I use a ballcock in the tank. As a backup, I use one of these:-
http://www.garden4less.co.uk/hozelock-ac-pro-water-timer-with-rain-sensor.aspIt says £93+ as RRP but you can pick them up for a lot less (about £60).
Obviously you don't set it up with the rain sensor.
Unlike most similar products, you can set it to be open for up to 24 hours - far more time than you will need to fill a 1000 ltr tank with a 40" RO.
What I do is, each time I restart the RO, I just set the time to somewhere more than I will need for it to fill the IBC. The ballcock will stop the flow first and during the time between the ballcock activating and the timer activating, you may get some waste going down the drain (though this often doesn't happen anyway I find). However, once the timer kicks in, it will stop the flow at the tap. I do find that I need a bucket underneath the timer and tap as there can be slight dripping at times after the timer switches off.
I use this method because I was once let down by a solenoid that had a bit of grit get into it. It resulted in flooding as the tank just kept overfilling.
I know this way seems a bit of a kludge but it gives me some sort of a backup in case of failure.
N.B. The timer runs on two large batteries that will need to be replaced periodically. I've been using this setup since November and the batteries still show as fully charged.