There is no problem with runway availability, surely the problem is one of airspace, and that is already full ! You can use or even add as many runways as you like in the SE but will it really make a measurable difference ?
In addition airlines will not dilute costs over two airports, hence the requirement to ram as many flights into Heathrow as possible.
Some years ago BAA offered a not inconsiderable 25% decrease in landing costs to airlines wishing to switch from Heathrow to Gatwick.....the number of airlines who took up the offer was nil so using Manston , Southend or indeed Stansted is not an option....although clearly something does have to give.
For years and for a variety of reasons "total UK demand" has by and large been forced into the South East, there is no incentive or appetite to do anything different, there are too many vested interests BUT something does have to change, we cannot carry on like this. My personnal belief is to develope Manchester, or start again with one airport and 6 runways.
Manchester is an airport located at the centre of the UK, its within an hour of many of the UKs major cities, it has excellent connections by road and rail and high frequency connections to the rest of the UK and Europe, these are already in place. With two runways three terminals Manchester could handle another 34m per annum and could even be expanded much much more ......
I do however fully take on board that an almost embarrasing number of airlines have tried and failed at MAN so why on earth should we go down this route again, well basically because going in the other direction and expanding in the SE is even less of an option...... something has to give !