They can be trimmed at any time, but the best time is in the spring or about now...

Good advice but if you're going for once a year pruning now is the ideal time. Do not cut back into branches where there are no green leaves. They will not regenerate.

thanks for your response, its a great help

Use a chainsaw, about half inch from the ground. I find that works! Phil J

It's a trim not fell it

Agreed PJ leylandii are VILE!!!