Thank you all for your help. It's a pretty derelict ruin and they don't seem to have many artefacts so I think I will donate it to the visitor's centre.
Jwarner51: I don't have a detector, just mill around my local beach looking for interesting rocks and things. I have a handful of strange, unidentified found objects and, since you've all been so helpful, I'll probably post them here to see what on earth they are.