The 10.000 mustard gas bombs and 2,000 inert ones is such a precise amount that I wonder if perhaps they were for an overseas order.
In the mid-1950s I can think of many potential customers but a total of 12,000 bombs would by standards have had to have been moved initially by rail.
We can trace most of the mustard ever made in the UK and its eventual destruction but to lose 12,000 bombs is a big hole in our calculations. Someone out there must have knowledge of them.
It appears that at the end of WWII it was decided to retain a strategic reserve of Chemical Weapons. The scale of holding being one third of that held at the end of WWII but because of the wastage in stocks, primarily because of the fragility of the 65lb weapon, it was necessary to manufacture new bombs with mustard being held in FFD tanks. This was Operations Pepper Pot and Spring Onion and took place in 1953/54.
I still do not know the eventual fate of these munitions.