Thanks for the observation, the short head and dark edges to the scutes can just be seen providing enough detail to distinguish the species. Looks like it sustained an injury to the rear left section of its shell at some time.
Thanks Damian. Yes - noticed the nick - that's why I thought at first it was a WW1 helmet! There was a rusted metal drum not far away. I didn't want to scare it so I took just the one quick photo and left it alone.