I'd wait until it has finished flowering as mine flowers quite early can you prune it in stages ie over a couple of years so that it doesnt 'shock ' it too much maybe reduce its height so that it still has plenty of leaf left to feed it

mine has gone all leggy and lost loads of its leaves. it was about six foot ...now after ive chopped it, its about 4.....it wont have many flowers this year but hopfully it will thicken up in the summer...sometimes i think u have to forgo flowers and chop things back...

Thankyou Pamq and Sandra
Mine flowers later in the summer so i think I'll try taking it back in stages, starting when the weather warms up. The flowers are insignificant so I can forego them this year.

by giving a shrub a year off from flowering , it uses all its energy growing...;-)

It seems a good idea to prune it this year then..
Thanks Sandra