The first thing that I would check is the drainage. Is it still in the original pot? If so, has decorative foil, if any, been removed? If it is sitting in a saucer, the saucer needs to be emptied soon after watering, so the plant isn't sitting in water for long.
If it has been replanted, does the new pot have drainage holes? Is the new soil fast draining? I would use a commercial cactus mix, myself. Depending on temperature and humidity, you might need to water only once every 2 weeks, though, of course, it shouldn't be allowed to wilt between soakings.
Another thing to check is the light levels. Gardenias like high light levels, indoors, and several hours of direct sun a day is not out of line.

Thank you. The drainage is OK as it is in a plastic plant pot with holes and then that is within a 'fancy' pot. I will try watering every 2 weeks and hopefully that might do the trick. The position it is in is great for light and sun. Thanks again for your quick response.

I would look again at the drainage aspect. As it is a pot within another pot, perhaps when you water, it doesn't drain adequately from the main pot so the roots are constantly in water. I would try and put some material (small stones) inside the bottom of the second pot to lift the main pot up. You could also put it outside in a sheltered but sunny spot for the summer.

Gardenias like acidic soil. I use an acidic fertilizer.

I managed to kill the one my mother in law bought me, but I hated the perfume so perhaps it knew I didn't like it!

Thankyou all. I shall try the suggestions you have made and funnily enough, being at my wits end with the plant, I had already thought about putting it outside for the warmer summer months. It cant get any worse unfortunately as I tried not watering as much and I lost more leaves even quicker. Such a difficult plant, but so beautiful when they bloom.