Although raised in Bedfordshire I am sorry to say I've never been out to see this stone - but I have found mention of it in another website - kind of encouraging the idea that it does exist.

http://www.mysteriousbritain.co.uk/england/bedfordshire/bedfordshire2.html

might be worth getting hold of 'from carnac to callanish' to see if the grid reference is different to the one given by guttbucket? There's no sign of a stone on my beloved os getamap site, but then if it's only small it'd be easily overlooked by a map make from out-of-town. People in Marston Mortaine must know!!!