Welcome to GoY.
Firstly, lets address your bare bottom (hedge), this is usually due to pruning your hedge vertically causing the lower branches to be shaded out causing die-back. Hedges are best pruned with sloping sides to allow light to reach the bottom of the hedge.
Secondly, privets are thirsty plants and the soil beneath the lower hedge will be bone dry or will be in a couple of days, even after watering.
Thirdly, anything you plant under there must be shade and drought tolerant.
Sorry I cannot help further as I'm more into trees (bonsai) rather than plants/flowers.
However don't dispair, as there are plenty other users of this site who can give you plenty of advice. We don't always agree with one another but that's the nature of discussions.
Loads of help, lots of fun and good people too.
I hope this helps. If it didn't at least it gave you something to read! ;-)

Many thanks for that. My husband does slope the sides when cutting ao will try the 'water treatment'. Finers crossed

Oops! Apologies for the spelling mistakes on previous comment, was typing too fast.