Can this old buddleia be saved?

[DogBagKite]

I've had this a few years, it's never been great. I thought they were supposed to like growing anywhere and being ignored but this one hasn't thrived. Probably because it's in a pot. I can move it to the ground now I've had a reshuffle, but wondering if it's worth the effort. Any ideas or advice please?

To add, I'm in the SE with clay soil but in a newbuild so soil is a mix of clay and a lot of old builders rubble. It's a nw facing garden. They grow in abundance along the rail lines locally.

It's needs chopping down and put in a full sun spot. My neighbour cut their down to a tiny stump in spring and now it's 2x2 metres big! Not sure this is the best time of years to do yours now tbh

Thank you! No chance of full sun here I'm afraid unfortunately. It will get partial sun but that's all I've got. So do you mean literally cut it back to a couple of inches above the rootball kind of stump?

Don't cut it back now - just plant it in the ground and keep well watered. I wouldn't worry about builders' rubble, I've seen them growing out of walls. You can prune it back to about a foot high next spring.

Thanks both! I'll give it a go, nothing to lose really as it's not doing much as is.