Muddymike wrote:

On further inspection of the images I think you are right, The top/left
chamber may connect to the centre one further down, if so its certainly a
three stage septic tank. Ten rows of bricks down sounds high for the water
table. Is it that high outside the brickwork? If not it may be that the
chambers are still retaining water. If the rubble inside is loose it could
be the remains of the old lid structure. These were sometimes concrete slab
and sometimes corbeled brickwork the beginnings of which can be seen.

Keep digging you may find treasure.

No treasure yet :-) but a few more details emerge.

There would appear to be a solid base about 3'6" below ground level. At
least there is in the centre chamber so I assume there is in the other two.

As far as I can see - and remember it has muddy water at the bottom -
there is no obvious pipe connecting the left hand and centre chamber.
However there is a patch of brickwork at the bottom of the dividing wall
with no vertical mortar and winder than expected gaps. Could this be to
let water through? I don't know how septic tanks are supposed to work.

Finally, I dug out a bit of the lawn and there does not appear to be any
evidence that the construction ever extended beyond the front wall. I
was hoping I might find cut off bricks where the wall used to extend.

Any more ideas?

Andrew