Just to update anyone interested in this subject...
Rhinox Hide is not the same. I popped into my local GW today (for the first time in maybe 15 years) and bought a pot.
It's darker than scorched brown, and doesn't have the elusive red hue. Not quite as dark as vallejo charred brown, but closer to that than scorched brown.
So seems scorched brown still remains without an alternative!
However Vallejo Dark Fleshtone to me seems like a very good replacement for Citadel Dark Flesh. It has the elusive red hue, like a lighter scorched brown, so for what I do will be used a lot for blending flesh colours from dark to light.