Boyton Tank Range

This is reputably the Boyton Tank Range.

An article in Suffolk & Norfolk Life magazine (Dec 2000) is entitled: 'Lowestoft to Harwich - 20th Century Coastal Defence' by Alistair Graham Kerr gives more detail:

"At Capel St Andrew you can turn off and walk down to Boyton Marshes.
This was a tank range and has several interesting buildings. The
coastline down to Bawdsey from Boyton Marshes has a number of
pillboxes..."