I agree here. But must say that an engine is not a living being as such, they don't get addicted to ether, it gets used because there is a problem, and if the problem is not rectified, then it gets worse, hence using ether more and more.
Not absolutley sure, but would be surprised if the Leyland DAF 4 tonner did not have a heater plug, they work well usually, and the other thing to make sure is that both batteries are good. It only takes one of the pair to be below par, to give you slow cranking, after all a diesel relies on speed of cranking to create the heat in the combustion.
I recently rebuilt a 23C diesel in a Massey 35 tractor, now, 40+ year ago, people said they were addicted to ether and it was the only way to start them. Well this one only wanted a bit of heat and it was away, no problems, not bad for a 54 year old engine.