In message , Chris Hogg
writes

Something like this perhaps? https://tinyurl.com/y4ktvchk

Chris, brilliant. Whilst I have no idea what is below ground level, the
two parts above ground look exactly like those visible in the garden.

So, the answer seems to be, it is a soakaway for rainwater, installed
because, now being a new build, the rainwater cannot enter the general
foul water sewer.

Thanks all!

--
Graeme