is it okay to trim Leylandii at the moment?

I'd suggest leaving it until the chance of heavy frosts have passed, around April-May.

Now won't hurt most conifers. When you do chop Leylandii they need to be cut around 4ft below your intended height due to the 'feathering branches'.

On a similar vein, my escallonia are about to get their annual hack any time soon. Not ideal granted, but neither will it bother them, and I can clear the mess sooner which suits my schedule far better. . . . . . .