Nikon Australia gave me an answer within 72 hours - which I think is good service. The good news is that they can fix the 24-70, albeit at a price. I'll have that back in about a week.
The bad news is that my beloved D800 was already showing signs of corrosion. It wasn't economic to fix.
Back to the trusty D300 for the time being.
So the lesson learned from this - at a price - is that when you carry camera gear in a situation where it could get damaged, put it in your bag.
And by the way, the people I work with have pointed out (rather forcefully I might add) that things could have been worse. I was alone, out of sight and had a heavy fall onto rocks in a river. I could have been injured - or even drowned. I certainly have the bruises to prove that point, although they're nothing that won't heal.
All true, of course - but my D800 is no more!