I came across this last Monday, I was looking at Martello Tower 'P' and saw a mound of earth with what appeared to be some sort of wartime defensive underground bunker, having no idea what it was I took a photo of said structure [very much overgrown] which is situated to the NE corner of Martello 'P' and started to investigate
Here is what I found:
3.3 Royal Observer Corps Bunker
During the Second World War (1939-45), Martello Tower P was commissioned as a Royal Observer Corps post, meaning that it was manned by volunteers with the responsibility of watching for enemy aircraft. After the Second World War, during the Cold War period, the role of the Royal Observer Corps shifted away from aircraft observation and toward the detection of nuclear fallout.
The low grassy mound in the north-east corner of the Martello Towers enclosure conceals an observation bunker of the Royal Observer Corps. The bunker is essentially a concrete box, providing basic accommodation for four ROC staff for up to two weeks, in the event of nuclear war. It is reached via a small trapdoor and ladder: the only other surface sign of its existence is a ventilation shaft. This is of standard type and has subsequently been stripped of its fittings. It was installed in 1962 and decommissioned in 1968.