Lavenders do this as they age - expected usefully attractive life span is 5-7 years, by which time they're usually very leggy and woody. You cannot cut into old wood with these, all you can do is clip over the foliage. Reaching the too woody stage happens faster if the plants are not clipped twice a year, once after their main flush of flowering is over, and again later in around August/September, taking about an inch of the foliage at the same time.

thanks bamboo, what a pity if i have to lose it.

If you want to replace it, look in early spring, when small plants are cheap - they grow rapidly in spring anyway.

Or try taking some cuttings.