Improving soil in the border and hedge

You need to feed with some Growmore fertilizer which is granulated fertilizer use one handful per square metre approx and one handful of chicken manure as a pellet again one handful per square metre Here is a couple of links 
https://www.diy.com/departments/westland-growmore-universal-fertiliser-granules-10kg/1318588_BQ.prd
https://www.diy.com/departments/westland-chicken-manure-pellets-10kg/275513_BQ.prd
If you apply some under the hedge that will help create some new growth creating a thicker hedge with some new shoots 

Watering the hedge hedge with a dry line and feeding with some high nitrate feed such as Miracle-gro will help the hedge to thicken up