Clipping hornbeam and beech in late summer encourages the growth of sideshoots and new leaves which are then retained throughout the winter providing pretty effective screening as in the picture above.
If your hornbeam hedge is bare in the winter it could be that proper maintenance could produce the desired thick screen much more quickly and effectively.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
If the hedge is old and neglected it may need renovation pruning one side per year (maybe the outside face first). I have just done the second side and top this year on my Beech hedge. It looks awful currently but should bulk up this year with proper clipping. It looked like it had not been managed for 10 years. I cut it back to about 10" from the trunks, removing a lot of dead and crossing wood at the same time. It previously impinged on the path. I left enough room for new growth to be clipped back to a good face later. No doubt it will need a lot of additional work each year but this will improve over time as the hedge thickens.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.