That suggests there is a scale insect or greenfly issue. The sooty mould should wipe off as it is feeding on the honeydew secreted by the insects. how large are they Owd? a mild soap solution would help.

Very large! Sprayed against the aphids, but that still leaves the mould. We are talking 2 metres by 3 metres of Rhody by the way.

ah not realistic to wipe all the leaves then.
I suppose the soapy wash /spray would loosen the sooty mould then spray with water to rinse it off.

The most likely cause is cushion scale infestation rather than aphids - sooty mould at this time of year is a typical indicator. Best to treat for scale a bit later on and if it is scale, getting rid of those should get rid of the sooty mould over time.

https://www.rhs.org.uk/biodiversity/cushion-scale

First thing I looked for, but did not find any scale at all.