The instructions below are spot on for finding the stone(s), but I think Grid Ref SP782416 is more accurate than the one given.
Lying within a nature conservation area, the walk alongside the river is well maintained, and it's possible to see that the stones are resting on the surface rather than being embedded in the ground. That suggests to me that the stones are not that old, but I'd be happy to be proved wrong.
Park near the bridge between Stony Stratford and Old Stratford.
From the bridge follow the river down stream, that’s right if you are facing Old Stratford.
Follow the river for about 70 yards and you will see it right next to the path, there is another smaller stone next to it.
Stones of this size 4 ft x 3ft are rare in these parts.
We need some one over to check this out.
We could do with a second opinion.