Sorry you lost your old Buddlia,Saxonann,but it's usually normal practise to trim them back anyway,or to prune to quite low down.,ior you only get new flowers at the top the following year....If it's grown mainly for the horses to shelter under,I suspect that is immaterial to you..The branches are easily snapped,as they aren't as strong as they appear to be..Your old one would have probably grown again,unless it was uprooted by the Gales..good luck woth your new one,and welcome to GoY..

I've read advice given on here to prune back by half to prevent wind rock.
How far you prune back to prevent wind rock would depend on how much height you need to get the following year and how much growth it would put on in a single season in your garden. Telling us how tall is the fence it needs to arch over might help get clearer advice.
Personally, I need to keep mine small therefore cut it right back to around 10 inches when new growth appears in spring.

Hi Saxonann and welcome to GoY. As Bloomer states Buddleia are normally pruned back every year. Perhaps you need to look at something different if what you are wanting to do is create shade for your horses...

I usually cut mine back to about three or four feet and it grows up to about eight feet.

If not cut back Black Knight should reach ten feet eventually, but as you have discovered at that height it may be vulnerable to high winds. But few things grow tall as quickly if you are in a hurry! I suppose you could try to limit it to a single stem and see if it would grow like a standard.

I would like to thank the people who replied to my question about the tall buddleia which I lost in a gale. Scottish asks how high my fence is, it is just post and rail, approx. 4ft.6ins. in height. I wish now that I had re-planted the old root as it would probably have grown again. I have taken in all the comments and would like to thank you again. I will let you know how my Black Knight grows. Happy New Year.

A new one should get to that height very quickly, probbaly the first season.