The most likely explanation is your Trachelospermum (also known as Rhyncospermum) jasminoides has cushion scale - you can check for this by looking on the underside of the leaves. At this time of year, you should see tiny yellowish brown things clinging near to the veins on some of the leaves. The sooty mould is an indicator that this scale infestation is present.

Treatment for cushion scale is best carried out June or July, when the nymphs hatch out and are more vulnerable to pesticides. Spray with Scotts Bug clear Ultra, or if you want something that isn't systemic, use Bayer Sprayday Greenfly Killer, but with that one, you must spray the underside of ALL leaves, which often isn't easy if your plant is large. You may need more than one treatment.

Thank you for your prompt reply we will check leaves

I forgot to say that if you get rid of the cushion scale, you won't get sooty mould on the upper leaf surfaces any more...