Th e two bulkheads with a gap in between offer about as good a protection as your going to get. The tank can't physically move and it's centre of gravity is very low. At a static weight of 380 kg the C Channels are probably fine but at speed, where weight becomes a multiple of that speed, probably not. The Navara C Channel idea was very clever and as usual ahead of the competition.
I have a 400 litre flat tank in my Ranger that sits against the bulkhead. In previous years I've had non-cooperative insurance companies who either wouldn't insure it if it was bolted down and others that wouldn't insure unless it was, subject to an engineers report. I know a lot of the insurance companies have now caught up and it's much less of an issue but the easiest solution for me was just to have the tank sitting in the back.