Chris Hogg wrote:
[...]
I'd put some polythene sheet down under the grass to stop the soil
from slowly disappearing, and I'd keep it watered in dry weather.

I'm intrigued. We get this suggestion of putting a plastic film under
a lawn quite often in urg, and it always strikes me as a bad idea:
too dry in dry weather, puddle in the wet. No different from laying a
lawn on a concrete slab. I don't see that there's any danger of the
OP's soil disappearing underground if it's on the firm base he
describes. Has anybody actually done it, and observed performance
over a few seasons?

--
Mike.