In message , Chris Hogg
writes

https://tinyurl.com/y5ypz7dx

Looking at the link supplied by Chris, the bits sticking out of the
ground look exactly like the illustration of a Klargester Gamma (top
row, 4th from right) although there are two of them, both black.

I don't walk the dog the same walk every day, so missed the bit between
a large hole and all covered, but I seriously doubt there was time for a
drainage field to be constructed, and not enough space anyway. I think
a soakaway for rainwater sounds promising, and will look for other clues
when next going past - but would a soakaway have those two corrugated
bits above ground level?

--
Graeme