basil,
re the taxi times etc you make very valid points, as does beagle re the obvious compulsory purchase of land and houses to make this project worthwhile, not to mention 'green' issues. This proposal was I believe rejected quite a while ago, as T5 would not get the go-ahead with such plans in the background. To make this idea viable you would really need a terminal facility of some sort closer to the runway than the current existing and proposed ones are, so T6 anyone? very very unlikely.
There has also been a recent thread on runways in the south east recently, Alconbury has already been rejected at the most basic planning level, even though as a site it has some potential.