Scale insects of some kind. They are sap suckers so yes they could be the cause of the sooty mould on the leaves, though that is normally caused by normal greenfly droppings.
Spray with a soap based insecticide as they are waterproofed and the soap breaks that down so the insecticide can work.

Yes, they are responsible for the sooty mould, and yes, as Owdboggy says, its a form of scale, most likely cushion scale. There will be egg masses beneath the leaves too.

Best time to treat is June/July, when the nymphs hatch, and the best treatment is with a systemic insecticide, Bugclear Ultra should do it. You can use non systemics or horticultural oils, but you'd need to spray the underside of all leaves thoroughly, and the effect is temporary. As for the sooty mould, that should start to flake off on its own later in summer.

Thank you both. Much appreciated.