It has to be a runway at Gatwick in the short term, coupled with increase in stands and a full taxiway at LCY and a proper transport link to Stansted to help the Poor go on holiday.
Why build another runway (and terminal) on some of the most expensive brownfield land in the country, with significant engineering complexity (re-routing the M4) for a shortened runway which is closed 25% of the time and which faces massive political opposition both from residents of West London, a main user (BA) who would both refuse to pay increased landing fees to fund it and would lose out through reduction in the value of its increasingly monetised slot portfolio, as well as meddling EU environmental restrictions?
Not to mention the issue of where all the new employees this expansion would necessitate would be housed....?
Heathrow just doesn't stack up - a longer term solution which meets our needs to 2050 and beyond is needed.