Being a civilian, and an EOD bod at that, I'm a bit rusty on Unit Ammo Storage matters but, straight of the top of my head, I think the rule is:
"Personnel responsible for, or employed in the day to day running of the unit ammunition store must have completed and passed the Unit Ammunition Storeman Course at the Army School of Ammunition at Kineton, Warwicks"
Which may or may not be in Ammunition and Explosive Regulations Vol 2, Part 4, Section 2, para 10 (or thereabouts - I don't know, I don't have a copy). To me, the above says that, if you have ammo, you need an Ammo Storeman.
I'm sure (or I'd at least hope, given that there's a thread on this forum that asks 'Can we show the AT trade in a better light?', or somesuch) that, if I'm wrong, a real AT will give you the DS solution.