How far back and when should I prune lavender plants

Trim after flowering, best to prune in April but do not cut into old wood

You can do it now - cut back the flowering growths and take about an inch of the leaf growth too, to reshape them all over.

I have just trimmed off the flowering stems that have finished and then trimmed it by the merest fraction just to give it a little better shape. But a very light trim.

thanks for the info.