I believe this is the control bunker for a Starfish decoy site from WW2, I suspect the area was a decoy for the nearby Marske aerodrome which was used during the war.
I’m sure it will be fully explained when the relevent volume of Defence of the UK by the North Yorkshire and Cleveland 20th Century Defence Study Group is published.
The pipe on the site is stamped with the name of the North Bitchburn Fireclay Company, which was apparently the brick making subsidiary of Pease & Partners who owned the nearby Upleatham ironstone mine.
UPDATE : As of August 2010 this site has been completely flattened and cleared by the farmer.