Any container that has held Dangerous Goods is still classified as Dangerous unless it has been properly purged and cleaned.
CAP668 &
CAP675 as Bertie suggests might help, as might
this raft of stuff also from the CAA.
In theory you should have a Dangerous Goods Note for the items too.
If you can carry them underslung in a net, I don't think you'll need to comply with Carriage of Dangerous Goods by Air Regs. Just attach a 10ft medium weight chain to the load inside the net so that static will discharge on touch-down away from the potentially explosive contents.