FFF, to be honest never been to Little Rissington, but the drainage at Dunkeswell (200 foot + deep valleys to the North and South to provide drainage, with the runway at the top of the hill more or less) would lead me to believe that they aren't going to have flooding problems there* in the near future.
* Except for the potholes in the car park last time I was there, but that's a different story.