I planted 35 metres of 60/90cm bare-rooted green beech hedging in
mid-December, but have only now (early February) realised that I
should have cut back all the shoots by 1/3rd immediately after
planting. This cutting back advice came from various gardening books,
which I really should have read before planting the hedge! Should I
do this cutting now, or leave it until later in the year?

Advice will be much appreciated - preparing the ground and planting
was a major task, and I really wouldn't like to do anything which
might reduce the viability of the hedge!