Cliffe was always a red herring and was only there to divert the environmentalists attention away from Heathrow.(in my opinion of course)
Since land is so expensive and society will eventually encroach onto an out of town airport (look at the population that was around Heathrow in the '50's and '60's) the only real solution is to build an off-shore airport with an integrated transport system.
This would be massively expensive, but long term may well be the only real solution.
Fish don't form pressure groups or care about noise!