The transmitter is located at Orfordness which is in my own county of Suffolk, not Essex. Ironically the BBC World Service used to transmit from there on 648kHz.
Confusion arises because the Caroline ship Ross Revenge is anchored in The Blackwater Estuary which is in Essex. Even when broadcasts are made from the Ross Revenge the transmissions still emanate from Orfordness.
http://www.radiocaroline.co.uk/#648_am.html
http://www.radiocaroline.co.uk/#history_of_648.html