Looks like one of the slug and snail eating blue-black beetles.

I agree - was happy to find one in my greenhouse earlier!

If it's about 3 - 4 cm long, then it's violet ground beetle, which, as has been said, is a predator of little slugs. More power to its efforts!

Thank you all. I shall try to tread carefully as I would hate to squash this helpful fellow. He is beautiful. He will be my best friend if he eats slugs.