John Rumm wrote:

snip

Strictly speaking it is a cesspool (probably 4 x 5 x 8' brick built
chamber in the ground) which also has an outlet. The outlet then runs
off our property an apparently into the next door farm's Klargester. So
its main purpose in life seems to be to accumulate sludge, and allow
some decomposition to occur. Previous owners apparently only rarely had
it emptied, but I have a feeling with more of us living here it out to
be done more regularly. I shall have to go have a prod with a drain rod
and see what the sludge level is.

That sounds like a single chamber septic tank.