Mathew Newton wrote:

Actually, I think my issue is multifaceted and something of a perfect storm:
(Snip list)
.... I also wondered if your extravagant and well intentioned subsoil might
have made a tempting home for crane flies whilst being too clean to contain
any natural controls (!) - the law of unintended consequences.


I will then give a treatment of nematodes
Don't forget the low tech option - the more birds come to your garden the
fewer crane flies there will be, and the more leatherjackets will get eaten
too. Unfortunately by different types of bird....


Thanks again to everyone for all the input - it has been really appreciated.

Glad to help. URG was always usenet at its best.