Lavender does get leggy with age im afraid. I would take some cuttings. You could cut it down. But I have a lavender bush that is so old, its really leggy but it was my Grand fathers and I would never cut it down! But you must do what you want to to. If you take cuttings you'll have something to replace it with - if thats what you want!

You're not supposed to cut into old wood on lavenders - they should be trimmed over twice a year, taking a little of the new growth each time, to reshape, and after the main flush of flowers is over the first time, and August the second time.

Agree with Bamboo.

Sounds like yours is also in very rich soil and possibly in shade or part shade?

Go for a compact angustifolia or vera type such as Imperial Gem, Munstead or Hidcote and plant in full sun (if not already).