Looks like a heavy aphid infestation - if its a large tree, then spraying with insecticide isn't very practical, but that's what would clear them. Info about aphids, including treatment, here

https://www.rhs.org.uk/advice/profile?pid=181

Aphids go for soft, fresh growth, so you need to keep a look out for them each year, while leaf growth is developing, and treat accordingly. You can use insecticide, but I prefer to squash them or spray the undersides of the leaves - which is where you'll find them - with agricultural soap and water. (I've never seen aphids on mature hawthorn trees, so I'm assuming this is a young tree and therefore relatively easy to treat.)

Thanks so much, Bamboo and Rosierose for helping.me out here. Yes it is a young tree and I will treat it accordingly. Cheers. Rob