I don't expect to see Spirobranchus worm on the top of a baby crab so I turned it over and ...
there it was - Sacculina carcini - the parasitic barnacle, noted to be common in Green Shore Crab population, but apparently also in Edible Crab. The Collins field guide had this curious comment: "We only include this species for completeness as undoubtedly everyone in the British isles is already familiar with it". How very odd ... I guess I'm the last then.