Sorry I double posted this:(

Kev,

Powdery mildew sounds like a reasonable guess. It should not be that big a problem to correct. Bright sunlight, lower humidity levels, and good air circulation are helpful conditions.

One thing to avoid is regularly misting or wetting of the leaves. Also, clear away any dropped leaves from the plant and container as they can harbor the spores.

If the condition is very widespread, you can treat powdery mildew with a sulfur-based fungicide. Some of the usual manufactures of home/houseplant sprays sell premixed solutions that should work. Safers does. Neem oil is also supposedly effective, but it is a much more pricey option. On the positive side, Neem oil also is used for treating a number of other problems.

Good luck!

Don