Dangerous goods advice- UK regs and ADR

Hi Dave I’m seeking clarification on a couple of points.

I drive for a plant hire firm where we hire diggers out either operated or self drive. When they are operated sometimes we are responsible for supplying fuel (red diesel) this is usually in the shape of a couple of 45 gallon drums which travel with the machines.

The larger machines 13t / 20t obviously use more fuel and have 1000l bunded flasks, occasionally these get run out to a machine if it has been moved from a site where there is fuel to one where there isn’t.

Now I seem to recall it’s OK to transport a limited quanity of fuel with the machine, but how do we stand in this instance.

The other day I was doing a transport job moving someone elses machines, when I got to site the digger driver was doing his nut because his yard hadn’t sent a tank with me. The tank in question was a 3000l job, would I be right in saying that would be a definate no no without ADR.

Are orange boards and extinguishers a requirement when moving limited quantities.

Appreciate any help for future ref.