It's a nutrient deficiency. Try the epsom salt treatment. Add 1 tablespoon of epsom salt to a gallon of water, give to your gardenia to drink. It usually does the trick. Change won't happen overnight, but you will start to notice a difference.

Depends - if its just a few leaves and they've gone solid yellow, they're just getting ready to fall. If the yellowing is mottled rather than solid and widespread, including on new growth, soil's not acid enough - I'm assuming, as you're in the UK, you're talking about Gardenia houseplants, unless you mean G.'Kleims hardy', so if they're in pots, you can use sequestrene iron tonic, or, much better, repot completely using ericaceous potting compost, especially if they've been in the same pots for a while. More info here

https://www.rhs.org.uk/advice/profile?pid=367

You can also just throw some pine needles around the base of the gardenia or use any potting mix suitable for rhododendrons, camellias or azaleas.

THANKS to 'All' for Answers. Much appreciated!

I would add– I had this same problem with my viatnamese gardenia. I repotted with acid mix about a month ago and no change, if anything it is worse. Going to try Epsom salt!

Ooh, that one's barely frost hardy, I assume you move it inside for winter if you're in the UK. It does like acidic conditions...

You might find this video helpful.

https://www.youtube.com/watch?v=v_ZCyZt7WZI

Ah, if only, but if you're in the UK, it ain't gonna happen. We just can't have large flowered gardenias outdoors, never mind see them reach 4 x 4 feet. I wouldn't mind moving to Oz anyway though...