My guess would be insufficient light is finally catching up to it. iIRC, this is a full sun plant and as such should receive direct sun. Not doing so results in weaker, longer growth that is unable to support itself.

Thanks. I do have some material that suggests it needs as much light as possible but then goes on to say, apart from direct sun in winter. I was veering towards thinking that the sun it was getting (in the morning when the sun is in the right place) was too much and that for some reason, now classed as winter. I can try giving it more sun though, and see what that will do.

IME, very hardy (zone 4) outdoor plants like this make very poor houseplants. There is very little we can offer them in terms of indoor growing conditions that would be comparable to what they experience in nature. I would plant it outside and stand back to witness the improvement!!

btw, these will grow perfectly well in ordinary garden soil - they do not have to be in water or even a heavily saturated spot. They will easily tolerate being quite droughty if pressed.

I had to look up the equivalent zone for where I live in the UK. It would be zone 9. In any case I live in a flat so outside isn't an option.

No balcony, deck or porch??

Nope, none of those.

In the UK it needs every morsel of sun you can give it even outdoors. I'm afraid you are probably going to lose it if you keep it inside.

"In the UK it needs every morsel of sun you can give it even outdoors. I'm afraid you are probably going to lose it if you keep it inside"

... unless you provide supplemental lighting.

Well I can try and find some space on a south-facing windowsill.

It is not the same as being exposed to direct sunlight. Even the best grow lites do not produce the same light intensity or the benefits of real (not filtered through window glass) sunlight.

It did look pretty healthy for quite a few months before it began to droop.

Supplemental lighting for plants in the home is not really a thing in UK homes. I've never seen it outside commercial premises. Plus indoors it is likely to be too hot and dry for hardy outdoor species like this one.

It's not really a thing anywhere, really, except for those of us who want to keep our light-loving plants happy indoors when sunlight isn't available. That's just what we get for collecting light-hungry plants. Especially people like me with homes which don't get a lot of sun.

There's something addictive about watching a once-sad-looking plant perk up and start growing again after adding a cheap little high frequency light bulb.