Sinking feeling

No, not the boat! (Well, not as far as I know). For the first time in three weeks we've got a weekend to spend on the house, so while Jim was working on the bathroom, I carried on attempting to make some kind of impression on the garden. Tucked away in the corner was what appeared to be a laege stone slab, but on pulling it away from the wall and scraping out the mud, it turned out to be this:

I feel a water feature coming on.