Phil L wrote:
I'm going to have to build a soakaway, which will take surface
water
from a 55m2 garden...there are no drains in the area.
The subsoil is mostly clay, but it's at the top of a hill - what
size
should I dig the hole and what should I fill it with? - there are
no
limits to the size of hole, nor to the infill material(s)

TIA

For anything like this, you can't beat McCormack:
http://www.pavingexpert.com/drain08.html

--
Mike.