Hello and welcome, breuddwyd. Definitely aphids. I get the same thing here. Possibly grey mealy aphids, I think.

They are really, really hard to get rid of as they get into all the nooks and crannies of the plant. You need to make every effort to eliminate them early on as you will find them inside your sprouts when you come to eat them on that special day of the year that we're not allowed to mention till the beginning of December. (You'll get the hang of the mad rules here after a bit )

Me, I squish them and wash off the residue. A bit disgusting but it's the only way I can remove them for certain without using vast amounts of chemicals. I've tried washing them off in the past but they float on the surface of any puddle that forms on the ground and then climb back up.

My only other comment is that your plants look quite close together for sprouts. How far apart are they?