There was a fascinating programme re Porton Down on the Beeb a few mths ago that showed how complex these shells are to dispose of.
That’s it, fascinating but necessarily only skimming the surface insight into the place.
I remember exercising on Bramley about ten years ago and a ban on digging in was still in force (as it had been in 96, due to Mustard having been found by an exercising unit the year before). I'm amazed that a housing estate was built on any of it.
Hence why chilmark, Bramley and other such sites cannot be sold as they cannot be certified FFE.
The situation you describe is not negligence, it’s a legacy issue that is very difficult to resolve. The was an Op many years ago to clear Bramle, possibly Op Centurion.