Rare Original Sandbag Structure In Scottish Borders

Found this site on the Scottish National Monuments Record a while back, and made a mental note to visit it next time I was down there.
The old Dunglass road bridge outside Cockburnspath in the Scottish Borders was built prewar to carry the A1, the main road over the border. It was replaced about ten years ago by a modern bridge alongside. In common with the nearby Victorian railway, bridge, it had blocks in wartime, but these have been removed. What's really interesting is what lies under the Eastern end of the old road bridge.
Among the concrete pillars is a sandbag structure, and though it's precise purpose is unknown, it's clearly military. It's about 20 feet long by 10 wide, and one wall still stands about seven feet tall. I can't see it being a store, since it can only be accessed down a steep slope (perilously near to a sheer drop into the ravine), and there are no remains of steps to be seen. It would be a hell of a job carrying boxes up or down there in wet conditions.
Wouldn't be much easier walking up or down, but I think it's more likely to be a sentry shelter.