We didn't know what it was but thought it might be some kind of beetle. Book and internet searches revealed it as a Harlequin ladybird larva. This is an invasive introduced species and feeds voraciously on aphids but if these are in short supply it will prey on native ladybirds, consuming eggs, larvae and pupae and moth and butterfly eggs and caterpillars. It has also shown cannibalistic tendencies when food is scarce. It has been present in the UK since 2004 and is colonising rapidly.
Very smart in its black and yellow livery though, isn't it?
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Thank you to Misty Dawn for this meme.