Thanks for your reply - had difficulty trying to respond in Firefox 13.0.1!
Took the dock to an expert who said if it failed to connect with an iPhone (it failed), it would probably fail with iPod Nano 6. The Nano 6 also fails to connect to the Griffin iTrip, which charged the Nano 4. Both dock and Griffin have the 30-pin connector, but only the Griffin has a cable to connect the iPod. It sits directly in the dock. It does charge in a Goodman clock/radio/dock, which I had used with the Nano 4. So it seems to be a matter of luck whether charging devices which charged Nano 4 will also charge Nano 6.