Lavender can't be cut back into old wood. Usual procedure with lavender should be to trim them back a couple of times a year, taking the spent flower stems and about an inch of foliage each time. If that's not done, the plants get leggy old wood on them, and need replacing, so if your daughter's ones look leggy and woody, its probably time to replace them.

Thank you Bamboo. I'm hoping they haven't reached that stage quite yet, but will take a closer look at them before doing anything.

Take lots of cuttings and root them in a gritty compost, they are quite easy to root.

I gave mine a good tidy up with garden shears, making sure I didn't go too far into the older wood .... just a decent haircut!