I've had exactly the same problem in the past when I've had to break the pot. The only way I can think of is to lay the pot on its side and train a hose onto soil which may eventually wash all the soil out leaving just the roots which can then easily be removed.
Roy

Me too. I tried to use a long carving knife to cut around the edges but had to break the pot in the end. Next time, perhaps plant in a container with straight sides, but easy to be wise after the event.

take a knife vertically down from about 1-2" in from the edge. and work all around the pot and then push from the drainage hole. you may be lucky and get it out. the plant will recover given time.

Will have a go tomorrow. Thank you all for your advice. 🤞

Old bread knife is perfect for this job. .

I had this type of pot once and to get the plant out I gradually scraped the soil away with a small trowel, until it loosened the plant and then it came out.

Done! I was dreading it, had put it off and off and am pleased to report it was a lot easier than I anticipated. Disappointedly the plant was not rootbound as I suspected so not sure why it was flagging? Had fed it regularly. Next question is can anyone recommend a shrub that will last a few years with some heigh5 and would be happy in a pot?

Well done! Anything will be happy as long as it doesn’t have very thick roots like roses. How about a dwarf fruit tree, or a climber on an obelisk? Clematis from the Boulevard series? Or for something evergreen, a dwarf Pine. A tall evergreen grass? So many options!

Nandina domestica or Stipa gigantica?

Nandina domestica would be a good choice and wouldn't break the bank!

Thanks for your suggestions. Quite like the idea of a Stipa Gigantica as already have a nandina domestica.