Hi nickman. I think you're missing the point. The development of Manston as a commercial airport is being used as an excuse to improve the infrastructure of the area in which it is situated i.e. better road and rail links are being constructed on the basis that the airport will need these things "when" it gets bigger.
It's now reasonably obvious to anyone with half a brain-cell that Manston is a no-goer. However, this was obvious years ago. The problem is that nobody in a position of political authority will admit it, because they've all bought into the dream and agreed to spend hundreds of millions improving the infrastructure.
You (rightly) point out that this is just going to go round and round in circles. But this is exactly what the politicians want. They don't care two hoots whether it actually succeeds or not; so long as they get the infrastructure improvements. They still haven't realized that better transport links work two ways, and that improved road and rail links will simply make it quicker and easier for local people to get to the proper London airports. The real loser is the taxpayer, who (once again) is paying for another road to nowhere.....but this time there's a rail link to nowhere as well.