Mtuba2
So ... what has been the conclusion to this mystery???
We had a bit of a dig yesterday, dug about 6 inches down and 4 inches under and we didn't hit anything hard. It is about 15cm thick though, so when we tried to lever it up our metal spade bent backwards. Oops! I've told the builder because our plans are awaiting engineers reports at the moment, so I suppose we just have to wait and see what they say. But I'm pretty sure it's just an abandoned lid now. Fingers crossed!