Posted: 22/04/2016 at 16:31
Hi Peter - hawthorn's pretty robust, so if there's no particular reason for it being sparse, you could add more to it by getting bare root whips to put in the gaps. The bare root 'season' has just finished, but around September time you'll be able to order them through one of the many good online retailers for November delivery. Clipping hawthorn should normally keep it quite solid, but a good feed in spring and a mulch to retain moisture will also help.
Alternatively, you could get something else to fill the gaps. Hornbeam or Beech aren't technically evergreen, but the foliage stays on during winter. They are happy to be kept quite narrow and only need clipping once a year so they'd fit in well with hawthorn.