Looks like a type of scale insect. They can be scraped off using a finger nail and ones all clear use a soap solution to prevent them coming back.

Thanks for the advice but I think its a bit too much to scrape off with a finger nail. The plant is about 7ft tall and is pretty much infested all through.

Yup - scale insects - and they're pretty much impossible to irradicate completely. I'm organic these days - I had some Pyracantha badly affected a few years ago and rather than spray with pesticide I decided to rip the shrubs out instead. THe sooty stuff you're seeing is a type of mould which grows on the honeydew (a nice word we use to refer to bug poo!!) If you decide to spray, do so late in the evening on a very still night - that way you'll kill as few as possible of the beneficial things like bees and ladybirds, although you'll still probably find dead ladybirds in the morning.

Thanks for the advice