Recently bought a house built in '59 and found this large (~7 ft x 4 ft) concrete pad in the garden. It's located at the end of a path in a back garden, next to what i think used to be a washing line (now covered in ivy).

There is a small copper pipe protruding from the concrete, and i can put a stick pretty far into it, it's full of pretty musty stinky old water (picture 3). I've dug around the side of the concrete pad, and it's only a few inches deep. I took a couple swings at it with my sledge and it seems like it would be easy to break up (picture 4). The only other clue was these little decorative looking log thingies that look like little semicircles fixed around the base that were fairly rotted down.

Is this just the base of an old shed? Am I good to just go to town on it with my sledge and get rid of it? Or could this be the remnants of an old oil / septic tank or some other horrible thing that will make me wish that I hadn't bothered? The extra space in the garden would be great if i can get rid of it.

I'm 99% sure the drainage/water supply is at the front of the house and not the back garden, where this is.

Thanks for taking the time to read this as I don't really have anyone else to ask. The RICS surveyor I hired wouldn't tell me anything.

What is this & can I smash it?