Depends which Buddleia it is - if its B. davidii (lilac-like cone shaped flowers in shades from white to reddish lilac to purple) it should be cut down to within 2 inches of this year's growth in first week of April each year. If its B. globosa (yellow ball shaped flowers about now) this should be pruned after flowering. B. alternifolia, which looks quite different, bearing long branches with small flowers, should also be pruned after flowering.

Thanks, Bamboo it's still in bud at the moment so I'll have to wait and see!

B. davidii blooms at the ends of the branches, while B. alternifolia blooms on the sides of the branches.