colin haggart, it looks too high (asking for trouble!) to have been purely an air raid shelter. I would think B/H would have had a district heating scheme with a large central boiler house generating HTHW (high temp hot water) elsewhere. This could well be a sub station feeding calorifiers, where the temp. is reduced to LTHW before entering the pipes & or radiators that are heating the barrack blocks, etc. Small chimney for ventilation.