I'd disagree with the previous post. Firstly, you've chosen a difficult shrub to propagate by cuttings. Your chance of success is limited but you can only accomplish this in the early spring in a very narrow window time wise.
The new growth of branches is your guide. When these have reached no more than 3 to 5 inches in length, you may begin. First, prepare a container of a rooting compound. I live in the western U.S. so any brand names I give you may not be available in the U.K., but your garden store will recommend a good one. Then cut as many branches as you need for new plants. Cut with a good, really sharp knife as opposed to shears which can crush the cell structure of the branch.
Then, immediately dip the cut end in the rooting hormone. Some gardeners like to put the liquid rooting hormone into a fine gardener's talc to make it stick longer to the cut end.
Then, have a container for each cutting filled with very fine sand. The sand should be quite wet, but not enough to make it liquid. Place the cut end several inches into a hole which you prepared (so as not to rub off the hormone treatment). in the sand. It's really important that you not allow the sand to dry out.
Contd.