Leech in boot?

We had done a rather wet trip underground. And that leads to wet boots. It's nice to get them dry again for the next trip! So, quite a while later (bad idea), I took the insoles out to make the drying process go quicker. And I found a small creature on my sole! I first thought it was a slug. The boots were in the conservatory and that does host its fair share of slugs. But the creature was too sleek, too black, too shiny. I had another look and I wonder if it was a leech! I did not do any details taxonomic investigation; instead I evicted the invader. But I think it was one! And I hadn't seen one before in Wales...