Rivet Joint
The airport operator never sold the Royal Mail building site, they never bought it.
When the complete site was purchased from Nat Somers by Gazeley Developments it was broken into three sections. First was that now housing Royal Mail, Mercedes, Premier Inn etc, the present airport site, and the area to the NE in vicinity of the "SAM" VOR (when I was a lad runway10/28).
When BAA bought the current site they initially chose not to buy either of the other two sections. They subsequently purchased the NE corner when Gazeleys worked out it would be a nightmare to develop due to access problems to the site. Subsequently along came the Airport Master Plan with the intent to site their new Terminal Building in the NE corner, having missed the boat with the obvious site for it!