Sea slug - sea lemon

My 2 sons and I were rock hopping round the shore between Parton and Whitehaven and came across what we instinctively called a sea slug. Excited to find a new creature we stopped for a good look. Size about 6-7cm, oval shaped, big skirt, leopard spot coloured, good camouflage on the rocky shore, on the lower intertidal zone, soft but tough body without any shell. A marine biologist contact I am lucky to have identified it as a sea lemon. They only live for a year and are common in the UK on rocky shores.