This is a perennial plant, dying back in the winter, but as far as I know, it's only hardy in Zones 9 upwards and Britain is mostly Zone 8 or 7, so there's a real risk it'll die outside over winter.

Thanks very much for your quick response. Yes, I know it's frost tender and it will be over-wintered in a heated greenhouse so hopefully it will survive! However, the leaves have been yellowing and falling off for some weeks now and I wasn't sure if that was just it's natural habit as the temperature drops or whether something is wrong. It's also flowering and growing new shoots!

When you say a large pot, how large? Should be 2 feet deep and 12 inches wide as a minimum, really.

It is!

Okay, then inspect the plant thoroughly for signs of infestation or disease, and in particular, bits nibbled out of leaves. If nothing, check the compost for invaders, specially around the roots.

OK, will do, thanks!

Found some tiny (red spider mite size) black 'things' on the under side of the leaves and have now sprayed with bug blitzer. Fingers crossed this will halt the problem! Thanks very much for your input Bamboo.