Mr Red,
Shoeburyness was (I think), going to be closed and become part of the development, which also included a container port, high speed rail-link and new towns to house the airport staff. Stansted, Cublington, Nuthampstead and Foulness were the sites studied by the 'Roskill Commision'. The whole thing became a victim of the mid 1970s oil crisis.
The airspace around Clacton wasn't as complex back then of course.