This site is of disputed antiquity. If you have any information that could help clarify this site's authenticity, please post below or leave a post in the forum.
Sandstone, very weathered, probably due to effluvia from the nearby, but now defunct, Blyth power station.
The stone is on the NE edge of the caravan site. Nearby beach has many unusual lumps of rock, and the erosion of the coast may throw things out of the fields onto the beach.
In terms of disabled access, they don't come more accessible than this, it's tarmac all the way.