Looks like aphids to me

Agree those look like aphids. You can get rid of them. Make a 50/50 solution of dish soap and water. fill a spray bottle and spray them. If you just left them, eventually a lady bird or wasps will show up to eat them.

Thank you both, will try it. Though they may be eggs. They don't move & look more like eggs :(

Acer leaf gall mites?

I think that Longleaf has found it. If so, control isn't really necessary, though, if you can't stand it, or it is a very young tree, you can spray with bifenthrin--if that's available in the UK.

bifenthrin not for sale in the UK

Nuts! Some other miticides might be effective,but I don't know which are legal. Look for those labeled for eriophyid mites. Neem oil might be effective, but that is iffy.