Well, I made the decision, and I am stripping the FF Transfer box.
I cannot get the front propshaft off, even with the correct C spanner so this will have to stay on for now. The rear drive flange nut was tight enough, and took a great deal of effort to remove, but that done, and the speedo drive extracted I undid the bolts holding the rear casing onto the main body. Which reveals this
The tooth wheel and brass elbow are the speed sensor for the ABS.
On inspection there's some nasty gunk in the housings, and some little bits of rust on none bearing surfaces, so a good clean and it should all be OK.
Now how it works - the output from the gearbox leaves on the splined shaft shown above, this drives the centre of the epicycle gears and the rear output flange. The output ring of the epicycle drives the outside of the VC. This has an internal spline which drives the large shaft over the gearbox input shaft, down to the chain drive, and built up without the casings looks like this.
Firstly the output shaft to the chain drive for the front propshaft
Viscous Coupling with epicycle on top
Thats it, simple really - can't understand what I was worrying about.
Next a good clean, lubrication and reassembly after making some gaskets