This is a much overlooked columnar tree. I like it because it will remain branched to the ground at maturity if desired. Suggest that you repost this question on the Trees Forum. Overseas contributors may provide some helpful comments.

I planted it successfully, 8' apart, to provide privacy in a very limited space to a new golf course tee that backed onto a major roadway. A dumb design. Another situation when those doing the planning think that plants will cover up their mistakes. Don't get me going on that subject!

Thanks for the suggestion. Will repost.

When plants were invented, it's not as if The Great Creator assigned a proper spacing to each one. A "proper" spacing is determined by man in light of how useful he can make a plant be, in order to solve his needs. Therefore, it is the situation or yours and the site conditions that will guide one in determining an optimum spacing for the Hornbeam. Though you describe briefly some of the conditions, it's not clear how they exactly are, and a picture (that is not a close-up) will help those who cannot see it in real life, understand. We don't know what plant a "hedge row" is, so how the shade of the Hornbeam will affect it cannot be determined. Need that picture for more information.