Had thought that only another 10% could be extracted from mixed mode, so it came as quite a shock.
I would agree that 25% is absurd - the conventional wisdom seems to be that 15% is about the maximum that could be achieved:
"The penultimate scenario is the application of full capacity mixed mode operations. This cannot be modelled using statistics derived from current operations as the expected increase of around 15% in capacity and the much changed operational procedures cannot be reasonably extrapolated from the current situation. For this reason, the delay curve and validation of the statistical approach have been performed using data obtained from NATS HERMES simulation, which gives the only accepted prediction of mixed mode operations at Heathrow."
Reading the article in the Independent, the mythical 25% seems to have been obtained by simply doubling Gatwick's capacity of 54 movements/hour and comparing that to Heathrow's current maximum movement rate of 87/hour.