It is indeed Shoreham Ranges.
You can still make out the target butts on Google aerial view, and there is an “explorer “ video on the net showing that they still have the target raising mechanism. I have a photo of the butts at Bisley circa 1980 before they were updated, with pretty much the same equipment.
The mapping that I use is simply OS maps on nls maps, then select where by town, then scale, then by clicking on the box on the background map, a selection of available maps is displayed on the right, decide on date by scrolling down, a quick click and then zoom to the feature you want.
The Fort Halstead detail that I found matched MartinR posting exactly.
Over to you JW.