No interactions present.
David, an old post I know. But I am looking for the Hydroid Hydractinia echinata which almost exclusively resides on the shell-homes of Hermit Crabs. See http://www.habitas.org.uk/marinelife/species.asp?item=D3350
I suspect you made a mistake when looking for the Latin name.
What you show here is a Sea Urchin shell. It is VERY likely to be the common Sea Urchin Echinus esculentus but it is impossible to tell from the picture. Normally they are beautiful but fragile shells when empty - see http://www.seawater.no/fauna/echinodermata/esculentus.html
You have moved on a bit since this post but I have taken the liberty of placing a new ID because yours was shown as Likely which is not the case..
THREE MARINE PROJECTS
Lat/Lng: 53.784, -3.0596
OS grid ref: SD302324