In message , fumier
writes
Ooops, too much Agent Orange on Sunday nights! I think I used B&Q's
aphid killer. Any serious suggestions, please? "Soot" seems to be
spreading

You can clean the soot off and the honeydew it is growing on with any
simple wetting agent and water. The problem is some waxy plants like
holly and bay that get sooty moulds do not like losing their waxy
coating and will shed their leaves if cleaned too vigorously.

The sooty mould is only cosmetic on the surface it doesn't harm the
plant.

Regards,
--
Martin Brown