Pax Britanica
Think I have answered my own question here:
"Between 1991 and 1994 a series of changes to the northern and western boundaries of the Borough were made. In 1991 changes to the western boundary with Windsor and Maidenhead resulted in all of the Wraysbury Reservoir coming within Spelthorne and other changes in the vicinity of the M25 Junction 13. In 1994 the Cargo Terminal of Heathrow and part of Terminal 4 were transferred from Spelthorne to the London Borough of Hillingdon and various changes were made along the boundary with the London Boroughs of Hounslow and Richmond upon Thames. In 1995 Poyle and Colnbrook were transferred from Spelthorne to Slough"