Blackfly, probably. You can get bug killer sprays, but try washing them off with water & liquid soap first, as bug killer is not good for bees and other beneficial insects. Or - if you're not squeamish, wear a plastic glove and wipe them off by hand.

If you do use a spray, then do it either first thing in the morning or late evening to avoid other insects.

Thanks for the suggestion but I don't think that they are blackfly as I had them last week! They don't have the same body shape as blackflies. They are nearly round

You could still try to wash them off or spray them - sorry I can't help with an ID of the little critters.

Thanks anyway