Tell me about a leech I found.

Good morning everyone,
                                    I found it stuck to a piece of black plastic covering the garden furniture.  It's a dark green/brown colour, at rest about 3" long but 5" when exploring.  I'd like to return it to the wild, but where?

I did find a few leeches feet down in solid clay when digging the foundations of my shed/workshop.

Cheers and thanks in advance.  Tony.