The key area here is the amount of diesel your drivers are proposing to carry.
The carriage of dangerous goods legislation (ADR) stipulates load thresholds of dangerous goods that can be carried before the full legislation applies. Diesel has a load threshold of 1,000 litres before the full legislation of carrying dangerous goods by road applies.
However, the diesel will need to be carried in UN approved containers and the driver would require awareness training which includes what to do when a spillage occurs, correct stowage etc.
A 2kg dry powder fire extinguisher would also need to be carried on the vehicle. If the diesel is permanently carried in the van it would be advisable to fit a class 3 red diamond label (flammable liquid) on the rear doors and removed when no diesel is carried on board.