Pruning Lavender. Left alone Lavender becomes voluminous and leggy which might be fine if that suits. Alternatively it can be shaped with an annual trim for a more formal look e.g. when used in a Lavender hedge. 

 The important thing to bear in mind is that Lavender becomes increasingly woody over time. It doesn't grow back from the wood so only cut the uppermost (i.e. green) section section of each stem. As long as the cut is above the woody part of the stem the green will re-grow.

 Traditionally the time to prune Lavender is late summer or early autumn though some say spring is ok. Perhaps now is a bit late for me to be pruning but then again it's been a mild autumn with both sun and rain to extend the growing season. Sometimes you have to do things when you can though it's important to work with rather than against seasonality.

 NB the standard advice about pruning the green and not the wood is not quite as hard and fast as it used to be. Some gardeners note no adverse effect when they cut into woody stems as long as the cut is made above shoots of growth. Even so I think Lavender is one for a light trim rather than a hard prune.