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I can see where you are coming from, with the way those horns are pointing in. How long was that piece (it looks incomplete to me)? I'm tempted to suggest that it might be another S. canicula/S. stellaris case, one that has lost its tendrils. Small-eyed ray cases have more of a keel (a thin membrane running down each side). Check out the Shark Trust guide - http://www.sharktrust.org/shared/downloads/great_eggcase_hunt/eggcase_id... - for more images if you haven't already got it.
To find out about what's going on in the south west check out http://www.ispot.org.uk/forum/8409
Lat/Lng: 52.4814633, -4.051972
OS grid ref: SN607890