I have recently been asked if and how we should mark our works 4x4 when transporting 2 x 13kg propane gas bottles and 2 x 10lt methanol fuel cells to some of our remote monitoring sites. This is not a subject with which I have any experience in so any help would be welcome.

No need to mark your 4x4. Derek propane is class 2, methanol 3 with sub risk of 6.1, not that it makes to much of a difference. Store upright, secure, 2 kg fire ext, make the drivers aware of what they are carrying and what to do in the event of an emergency and crack on.