According to wikipedia some plants are badly affected by juglone and some
not at all affected.(No examples of the latter are mentioned) Does anyone
know of anything in the 'not affected' category that might be usable to
patch the hedge?

This article lists quite a wide range or trees and shrubs tolerant of
juglone. https://tinyurl.com/yb2dsy7m I've not read it through in
detail, but I imagine that at least a few of them would make
reasonable hedge plants.

Brilliant! Thanks for that - it has just provided a sensible use for the
wild plum suckers at the other side of the garden.