you can cut it hard back down to a foot high. it will resprout from the old wood too.

I have done this a few times in the past when I didnt get round to it the year before.

go for it :)

Hmm, that's a good question! I'll admit that what I do (coming across many unpruned for years Buddleias) is take it back down to about 3 or 4 feet and keep my fingers crossed, but I usually only do that if I can see signs of growth lower down. Inspect the bottoms of the branches - if you see new leaflets emerging, you can cut back to there or maybe a couple of leaf joints higher up. If you're not sure, wait till late March to do it, give it more time to burst into growth.

I give mine a light tidy up at almost any time and then in early spring I cut them back to about 2 or 3 buds or growth points from the base.

That's great guys! Thank you so much for your advice.

I've found you can be as ruthless as you like - they're almost unkillable!

I tend to give mine a good trim in Autumn after they've flowered,in case the stems get broken in winds etc, then I give it a good hard cut back in early spring - maybe to a couple of feet, cutting back to a decent bud.

....but I'm no expert ..... :)