Simply expand Gatwick and let that solve the problem in the south east. No need for an extra runway at Heathrow. Why have flights to Murcia, Alicante, Mykonos, santorini and so on from Heathrow taking up slots that could be used for business routes connecting the uk to the world? You know the ones that we are apparently lacking that only a third runway could provide. Keep the leisure at Gatwick, which after all it is what it is good at as you say.