Don't think it's a well shaft or manhole now as there are no services running through the rear of our house and it backs on to a playing field. The site was on the ground of an old stately home that was sold and demolished to build the estate. I believe our house was on the site of the old swimming pool. We do have a fox family living in the bushes at the rear of the house so I would say this is probably fox wee. It is starting to grow back now as there are a few green blades appearing.