I always prune after they flower, so with this species that would be May time (depending on the spring).
Its an established shrub so you wont do it much harm by reducing it size by a half (or even more if needed) it'll just look a little tatty for a couple of years until it can regrow a bit, plus it might not flower again for a couple of years.
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Thank you everyone.
As the consensus seems to be to wait until spring, I will hold back until the warmer weather.
I always prune after they flower, so with this species that would be May time (depending on the spring).
Its an established shrub so you wont do it much harm by reducing it size by a half (or even more if needed) it'll just look a little tatty for a couple of years until it can regrow a bit, plus it might not flower again for a couple of years.