Seat Leon intermittent rapid rise in engine speed

This has so far only occurred while the weather was cold, around 2 celsius, though that could be red herring. While accelerating hard the engine speed suddenly increased with the rev counter needle going well into the red. It took a second or two for the engine speed to drop and the accelerator pedal to rise. This happened on journeys last Wednesday evening and Thursday morning but not later on Thursday or on Friday when I last drove the car. It's only 11 months old.