Filling the gaps (Hawthorns)

Thanks for the hints! They are now too tall to give a ‘haircut’, unfortunately we’ve left them too long. Think just planting more will be the way to go.
No seedlings, ever, Veggie, no idea why that would be.

You can coppice hawthorn, so it is never too tall to cut. You can take it right down to the ground, or whatever height is required, and it will happily regrow.
As for no seedlings, I believe the seeds germinate best if they have been through a bird first, so look in places away from the hedge where blackbirds in particular perch.