"Tom"
I noticed yesterday that one of my two gardenias have a sort of white
coating on the leaves, they seem to be small individual dots (looks like
dandruff lol). to touch they feel a bit greasy.

I suspect your gardinia, like mine, has soft scale on it. Mine is best part
of 30 years old, and been in its current position for 15 years, and never
before had a problem. Best tell tale sign of scale is ant activity.

Spray with White oil on a coolish day when no rain is expected, and all
dandruff will suffocate nicely.

The Lady Gardener