Pillbox?

I pass this structure most days on my way to work. It's on the edge of the MOD Pirbright Ranges at Deepcut, next to the road and I think it is an old pillbox. Though strangely, it has ESSO plaques on the side, so may be something to do with oil or petrol instead. Still, as one of the few easily accessible derelict structures in the immediate area, I thought I'd have a go at blipping it for SarumStroller's Derelict Thursday