Got the same on my Kilmarnock willows. I bet they're the same - rose sawfly from the look of them. I'm surprised to see them on willow, but it is a food plant for a wide range of moths and butterflies, so I don't doubt sawflies like it too! I just wash them off, or knock them off by hand.

These trees are prone to infestation by caterpillars - you can either pick them all off by hand and kill them, or spray with Bayer Sprayday greenfly killer - it works on caterpillars too. I guess it depends how much time you've got, Caroeden, as to which option you choose! As for id of the caterpillars, a photo might make that possible - or not.