I said:

There ought to be some research reports into this kind of thing - I
suppose.

Root barriers, that's the thing.

Well, I found this:

http://www.profengineering.com/root.htm

http://www.ewburrownursery.co.uk/dow...t_Barriers.pdf

http://best4garden.co.uk/page/15

http://www.hy-tex.co.uk/ht_geo_rb.html

http://www.boddingtons-ltd.com/fores...ot-barrier.htm

http://www.wreford.co.uk/root_barrier/

http://www.arbortech.co.uk/products/barrier.shtml

Etc.

I think depth of barrier might be quite key. Perhaps I need a 30" deep
barrier. That means laying flags upwards, not sideways.

Root barriers seem to have some hope of working.