I have lifted the tank manhole cover and it is circular, approximately 6 to 8 feet in diameter. The top part is a dome of brick construction, and give the 'water' level it is impossible to see the rest of it, although I remember looking at it years ago when the level was lower I got the impression the lower part was not brick but possibly fiberglass? (hard to tell really, very dark and smelly when looking it!) The tank has a plastic soil pipe vent stack which rises to approx ten foot.
What is the tank most likely to be given the period the bungalow was constructed in? Seems to small for a 'porous' (ie leaking) cesspool, but after searching around the web it does not seem to be a septic tank. The ground around here is a clay/flint mantle up to six foot deep with chalk underneath (if that has any bearing on it).
Thanks in anticipation.