Today I discovered these guys on my rhubarb hanging around the flowery parts. They look way bigger than aphids (2-4mm). Any idea what they are?
I don't know what the bugs are but they're probably just after the nectar in the flowers; you need to remove that flower stalk asap. Rhubarb should never be allowed to flower because it takes energy away from the rest of plant, reducing the number of stalks, the part you want. Keep an eye on it ongoing to nip out any further flowering shoots.
Looks like a harlequin ladybug. The beetle shape is of course familiar and the colour markings are not typical of anything except the huge variety of markings ladybugs present. Ladybugs go for aphids so they won't be chewing on the rhubarb as such; examine the rhubarb flowers to see if there are aphids busy there. If you have any other crops with aphids on try moving one of these beetles to that area and see if it says thank you very much.