The best time for pruning is in early summer when you should do the 'three Ds', which is to take out damaged, dying and diseased branches and shoots. Pruning later in the season, and especially in winter and the dormant period runs the risk of introducing silver leaf disease.
I'd guess that by restricting the trees to a hedge, you are cutting out all the fruiting and flowering buds. Does it flower in the spring?
I'd leave it to bush out, flower and fruit, and then cut it back. I doubt you will ever get much fruit on it as a hedge though unless you leave some larger branches with fruiting buds on them.

Many thanks for that helpful reply. Actually we bought the plants as for a hedge, I forget where from but a reputable nursery which advertised them as hedging cherry plum. I will try trimming minimally when the nearby tree has borne fruit and finished - usually by mid-July.
Thanks a lot francesteresa