There are no Clypeaster species in the European seas!! European Echinodermata Check-List: A draft for the European Register of Marine Species (part of "Species 2000") compiled at TMBL (Tjärnö Marine Biological Laboratory)
In the Atlantic there are only a few living species, eg. the Caribbean C. rosaceus and C. subdepressus according to Schulz (2006), but these two seem to have deeper food grooves.These two can grow up to 20-30cm.
Your specimen resembles C. reticulatus (L., 1758) from the Indo-Pacific province!!