Which gardenia?

Hi Bamboo . . . I'm afraid I don't have its variety, but the leaves are glossy, mid-green, and it is not hardy. It's in a pot, in ericaceous compost. Thanks!

You haven't said whether its indoors or out, but either way, they don't appreciate hot sun in the middle of the day, so that might be an issue. You're supposed to keep it damp, and water with tepid water, and out of draughts, such a fussy plant in the UK... amazing when you think the tender ones grow and flower outdoors in other countries.

Oh dear, I did say in the question that it's in sun, but omitted to say 'outside'! so maybe that is an issue. Thanks for your advice Bamboo.