Given to me as a Greenbottle Blue - the red rump came through last moult now making me think B. vagans but it has had a blue tint up to this point which is still present on its legs. I was fairly certain it was a Cobalt until this last moult.
@Flexzone You're right, it's certainly not a GBB or a vagans, but I don't think it's a P. cancerides either. Mine have always stayed a single color, no variation throughout the body. It's also quite stocky for a P. cancerides.