I think its sooty mildew and it can be removed by laboirously cleaning each leaf with water and cotton wool or a soft cloth. My camelias suffer from it.

Thanks Cammomile, I`m in for a long job then as the plant is about 12ft high and 4ft wide.

Check the underside of the leaves, near the veins - you may have an infestation of Cushion Scale, and that will cause this sooty mould to appear. If you can see the tiny things, or any remains of white egg masses, near the veins on the underside, best time to spray is late June/early July, as the new nymphs hatch out, using a spray which treats for this (Westland Plant Rescue bug killer for ornamental plants, is one). This pest overwinters on the plant if not treated, and Trachleospermum is one of the plants it affects quite frequently. When you spray, make sure you get the underside of the leaves as well as the tops (as far as possible, with such a large plant).

Yes Bamboo there are little white lumps which I hadn`t noticed, thanks very much for your help.

in the meantime, you can attempt to wash off the worst of the sooty mould from the tops of the leaves if you like -won't cure the cause, but dealing with that symptom might make it look less unsightly until end of June.