Damp beneath living room floor

[bobasd]

if its the pavement thenthe patch wasnt a gulley an your not allowed to mess with the pavement. could have been the old water service external shut-off chamber.

seeing th e plinth is so high - about 15" - maybe just leave in place or you could disfigure the brickwok behind the plinth.
if the pavement slopes to your house then the council should be asked to rectify it.

 

[foxhole]

The square patch was probably where the air brick is, outside level was raised so patch was left open to allow ventilation but this turned the square into a drain causing damp as rain entered thru air brick , this was then filled but a small slit left open allowing air to enter , but still lets the rain in .