John

I don't expect that ground water generally will be a problem.
Our house is almost at the top of a hill, so drainage of the garden
generally should be OK.
I am guardedly optimistic about being able to make something of the plot in
question.
As mentioned in the original post, the area has been covered - probably for
20 years - in plastic membrane, concrete etc. so there's been no opportunity
for the earth to breath or nutrients to be added.
If I can get 12-18" of workable material, with the top 12" being fair
quality, I'm hoping to make progress.

Phil