Hello Lesley 4,
It's well worth cutting back lavender regularly to keep the plants compact. Always remove old flower stems when they are past their best, by cutting them right back to their bases. As for the foliage, don't cut into the old, hard woody parts, just cut into the green/grey foliage. You can do this in late summer, after flowering or in March, or even twice a year. Just trim off about 3cm, or even less if you find there's not much foliage on your stems to cut. I would recommend giving it a trim right now. Unfortunately lavenders do tend to look sorry for themselves in the UK in winter!