DownunderGardener00, the plant is in an outdoor in-ground bed. The lower leaves, to the right of the photograph, appear as uniformly yellow. I would trace back the stalk(s) and see if there is anything "wrong" with the bark.
The reason why I say this is because if there is something wrong at the roots, the youngest tissue (growing tip) will be the first to show the effect.
If something is wrong higher up the plant (at an internode, say); then the effect is seen immediately above the site - first - then working its way upward.
The 'text-book' symptom of Magnesium deficiency is a dark green triangle at the leaf base. The shape of the triangle mimics the overall leaf shape; the symptom is highly visible on the lowest leaves of the Canary Island date palm - Phoenix canariensis
Magnesium (Mg) is supplied by Epsom salts.
Manganese (Mn) is a different element and its deficiency is expressed by a different symptom. The remedy is a different product.
Gardenia often shows symptoms of iron (Fe) deficiency. Typically, the leaf veins remain green while the surrounding tissue gradually turns yellow.

Thank you Ronalawn82...so once I have inspected the bark etc as you say and if there is a problem at the roots...is there any remedy to it or should I chuck it and start again?

DownunderGardener00, the short answer is I do not know.
Whenever more than one condition is changed at a time, it is difficult to isolate the single one that might be the problem.
If you probe it and the medium is too moist, you might try this. Take a piece PVC pipe and try to form a vertical hollow in the container. Do 3 or 4 of these where you can. You can pour dry sand or potting soil in the hollows and hopefully, this will 'draw' some of the moisture away.
You can tell that I believe moisture to be the problem BUT Mother Nature has often made a fool of me.

DownunderGardener00, I did not intend to post the previous message here.
I believe that the Gardenia might be showing a nutritional deficiency.

It looks pretty normal to me. Most of my Gardenias shed their lower leaves, usually at the end of the dry season. Feed them with ash/bone meal. Unless your soil is very alkaline (lime), it's probably OK. Top dress with leaf mold. Low humidity can be a problem - Ian.