Just as an update...
We did eventually hire a Chevrolet Captiva 4x4, which was a great car to drive around.
We didn't actually make it onto the F roads, but ended up having to drive on gravel tracks, which were probably pretty much the same.
The only problem we had, was that this car which is a 4x4, aka all-wheel-drive, got stuck drive through some mud.
I then remembered that you really need Differential lock, to be able to drive out of mud, as both front and back wheels on one side were stuck.
If only front OR back wheels had been stuck, it wouldn't have been a problem, but it was both right hand side wheels were stuck and therefore the left hand side wheels just kept spinning.
Just a word of caution. We didn't drive through a river, it was a larger puddle...quite annoying. But a brilliant local turned up and pulled us out with a massive rope!