Offshore Radio Museum:

"Unlike some of the other forts, Sunk Head was clearly well outside territorial waters and when the Marine Etc Broadcasting (Offences) Act came into effect in August 1967 the Government was anxious to ensure that it wouldn’t be taken over again by an offshore broadcaster.

On 18th August 1967 Sunk Head was boarded by a contingent of the 24th Field Squadron of Royal Engineers from Maidstone from the tug Collie, under the command of Major David Ives.The Fort was weakened by acetylene cutting torches and 3,200 pounds of explosives were set. On 21st August 1967 Sunk Head was blown up, leaving 20 feet of the leg stumps remaining."

History of the fort and photos of the boarding of it, explosions and remaining leg stumps at this link

http://offshoreradiomuseum.co.uk/page463.html

Video of the fort being blown up