Its not a tomb stone!

You learn something everyday, there are loads of these stone things around the London suburbs. I always wondered what they were for and wikipedia saved me today

It marks the point where coal being transported into London is subject to tax!

For the interested have a look at http://en.wikipedia.org/wiki/Coal-tax_post

As with Sunday's blip this marker is only visible in the winter as it is completely overgrown in the summer