The bricks look more modern than the house, but perhaps I am mistaken.
I did more digging this morning, and a bit of bailing out
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It has solid concrete bottom. Between that and the brick walls, that must explain the drainage issues.
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A smashed old WC (I think) has been used as backfill.
A previous owner had knocked down the outhouse which adjoined the main building (a separate structure to these walls).
Seeing as there seems to be no ingress of water into the structure I'm excavating, I am guessing this was one of those old fashioned hedge-hog/toddler murdering ponds, which they backfilled with waste from the demolition of the old outhouse in advance of having their own child.
I would guess the pond was built after the war, possibly replacing a bomb shelter which previously was sank in that area?