My RHS book says remove the flowering stems in summer.

after flowering as it flowers on last years wood. My brother only prunes out the stems that have outgrown the area he wants it in.

Buddleja alternifolia blooms in spring on the previous year's growth. B. davidii blooms in summer and early fall on the current year's growth. Yours sounds like the davidii type, Novice. If it was B. alternifolia, it would not have bloomed until next spring. Another difference is that B. davidii produces cone-shaped flower spikes at the tips of the branches, while B. alternifolia produces small, globular clusters all along the branches.

Many thanks for your comments. The plant does have alternate clumps of flowers, like an alternifolia and serrated, spear-shaped greyish leaves, but might be a completely different shrub, I suppose. Perhaps the best thing would be to see what it does in the Spring, and take it from there?

It might be an alternifolia that is confused by the weird weather, this year. I understand that many spring blooming plants are flowering out of season now.

Thanks, Tugbrethil, I did wonder whether that might be the case. I think I will just wait 'til Spring and see if it flowers again then.