Toby wrote:
It is, but sadly any soil in the area is topped with a thick layer
of concrete right up to the edge of the house.

Cut back concrete along the wall, dig trench, fill to brim with coarse
gravel.

This doesn't sound like a diy job. Not if you mean the application of one
of those terrifying massive grinders that pavement operatives use in busy
shopping centres to cut curbstones! How wide does such a trench need to be
anyway?