It's some form of fungal growth but I don't know which one and what damage, if any, it might do.

Thanks . Managed to find it online eventually it's called horse chestnut scale .

Scale insect, not fungus. They are little sap suckers that do only limited damage. Remove by hand if possible. I have also heard that a dab of alcohol will knock 'em down. Systemic insecticides can be used but are a bit unfriendly.

We use a paint brush and dab methelayted spirits on them.

The methelayted spirits worked on mine a few years back.

My Cotoneaster that I have in a pot was infested with these the other week and the plant was looking very sorry for itself, there was also a lot of what looked like soot on the leaves by the worst of the infestation. I stood for over an hour removing them with a stick, after which I gave it a good feed and watered it well and also sprayed it as there were some tiny caterpillars. It has picked up now but I keep an eye on it just in case.