Nick Maclaren wrote:
Implausible. You need remove only 1-2", and no realistic soakaway is
that close to the surface.

You will need to dig much deeper to plant herbaceous plants - an
abolute minimum is 4-6" and 9-12" is more realistic. Doing that
might disturb the soakaway but, with care, shouldn't damage it.

Thanks, Nick. Removing just 1 to 2 inches doesn't sound too drastic,
although it's quite a big lawn so it would be quite a task. The ground
here in South West Shropshire is what locals call "ratchety", i.e. full
of bits of natural slate. This hillside is like a pile of stones which
earth has managed to penetrate!

Eddy.