Oceancrosser, the question you would need to ask to get a better idea is what does Gatwick Airport need to do when switching to the northern runway in an emergency situation. It's been far too long since I was there to answer, but closing RETs and various other stuff was major, and you then need to consider the influence from what shut the southern runway in the first place. If planned, 15-ish minutes, if not, don't wait around......