It would be amazing if a decision is ever made; even if it is it will be reversed within a year or two.
I did a lot of work on the Maplin Sands proposal as a relatively junior number-cruncher; it was blindingly obvious that it was the best option, and remained so after balancing the environmental impacts of closing LHR in favour of Maplin Sands. But the Government of the day couldn't make its mind up and so we did nothing.
And have done nothing ever since, except patch new bits on to LHR usually about 10 years after the demand figure passed the figure the new patch was intended to serve. The great Charles Stuart (BA, Brymon) made a compelling case in about 1986/7 for a 3rd (2000m or so) LHR runway to remove regional aircraft from the long existing runways. I'm proud of helping with the technical aspects of that case, in a small way. The proposal came up against a brick wall of resistance to change, which still exists today. (London City came about as a result of his getting nowhere with the 3rd LHR runway proposal.)
Fast forward to 2003, and the £100m study into London airport capacity that was grandiosely and misleadingly entitled "The Future of Aviation". That kept an army of consultants, including me, rolling in s**t, sorry, gainfully employed for about 4-5 years leading up to its publication. Every aspect of the problem was exhaustively looked at, calaculated, weighed up etc etc. Every conceivable option was examined and kicked into touch. The end result, assisted by the 26 staff BAA embedded in the DfT, was to build another runway at LGW, STN and LHR, but now necessarily now, and not in that order, and, by the way, not at LHR until a solution was found to the air quality impact. Which it has not been, of course. This solution could have been, and probably was written on a fag packet as the inevitable outcome before any work was done. BAA ruled, and this was what they wanted.
(One of the best, quickest and cheapest solutions, a 2000m runway at Redhill, 100% privately funded, parallel with LGW with 8 minute overland transit to LGW North Terminal was kicked into touch smartly by BAA, aided and abetted by a complaisant NATS. It could have been open and operating by 2008/9. The way it was dealt with showed just how corruptly the UK Government really works).
The "Future of Air Transport" was put in a drawer and forgotten by the Labour Government the day after it was published. Nothing new has been added to the body of information about London airport capacity, except that as time goes on relocating the principal hub airport of the UK, as should have happened in the 1970's, becomes increasingly difficult.
So here's a prediction. Politicians will continue to bicker about it until the present system of air travel, ie charging down miles of concrete with larger and larger thrust supported contraptions running on fossil fuel and polluting the atmosphere, is replaced by something new and totally different, say in 50 - 100 years time.
The UK has not completely lost the ability to firstly decide on, and then execute a major infrastructure project. But it can only do that with interminable delays, changes of mind, political interference and so on; Crossrail illustrates all of that while being a fantastic engineering achievement. For the foreseeable future, the bickering about London Airport capacity will continue with facts, half-truths and lies from all sides, and LHR will grow at a slower and slower rate with small incremental capacity increases, about 10 years after they are needed.