Jon,
![](./images/smilies/spacer.gif)
That does sounds like the standard Viscous LSD
![](./images/smilies/spacer.gif)
(which is very good).
![](./images/smilies/spacer.gif)
With both wheels jacked up in the air, car in neutral and handbrake off
![Wink ;)](./images/smilies/icon_wink.gif)
you can turn one wheel and after a small quiet clunk and a little motion you should see the wheel on the other side turn in the same direction.
![](./images/smilies/spacer.gif)
Drop one side of the car down
![](./images/smilies/spacer.gif)
- and you can now turn the wheel up in the air, but you should feel resistance.
![](./images/smilies/spacer.gif)
Plate type LSDs need a fair amount of torque to make the plates engage and the diff lock up.
![](./images/smilies/spacer.gif)
With the rear wheels up in the air, and you spinning one of them you'd not expect the diff to lock up.
![](./images/smilies/spacer.gif)