My RHS book says that if it's the Moroccan / Pineapple broom (cytisus battandieri), which is a semi-evergreen shrub that flowers on the current season's shoots from early - mid summer, then prune after flowering in summer.
If it's cytisus scoparius, an arching shrub flowering in late spring on the previous year's wood, then prune it early to midsummer, immediately after flowering.
Only the cytisus nigricans is different - cut back all flowered shoots by about 2/3 in spring.