Part Piddock...
at an initial guess.
I'm not certain on the ID, principally because there is not a huge amount of shell to go by, however I suspect this is a large Pholas dactylus.
The fragment I have is 47mm across, so at a guess the full shell would have been 15cm long - quite a respectable size and capable of drilling quite some hole!
Some great images of Piddocks at www.idscaro.net/sci/01_coll/plates/bival/pl_pholadidae_1.htm
A fantastic (if a little overwhelming) resource by Amgueddfa Cymru / National Museum Wales at naturalhistory.museumwales.ac.uk/britishbivalves/
