After a little more research and some input from readers i’m now confident these are the remains of a QF decoy site called Greenabella. These sites lit controlled fires during air raids to appear as targets struck by bombs. This location was a civil decoy for Middlesbrough.
A reference in ‘Fields of Deception’ by Colin Dobinson idenifies a generator house and site shelter at this location.
Both have an offset entrance or a blast wall for protection
I think the larger structure with the holes for pipes and a chimney is likely to be the generator building, with the smaller structure the shelter.
Other decoy sites in the area also have unusual structures.