Are Ben and Ned immortal?

[Firetail]

I think they can live forever, but if someone killed them they would die.

[Cornflower MM]

I don't really know. They can't age - But they quite possibly COULD starve, if memory serves in The Voyage Of Slaves.

[Albrithr]

The way I always understood it was that they are immortal, they cannot age, but they can change.  This is their true benefit over the crew of the Flying Dutchman- they still need to care for their bodies, otherwise they could be malnourished (which the crew of the Dutchman certainly is after 100 years of not stocking with provisions), and they can be injured, or ill, which would result in a long recuperation period.  But no, they were told that they had to roam the earth forever, so it seems they cannot die.
On a sort-of related note, I always wanted Brian Jacques to write a story in this series that took place during Dunkirk/the Battle of Britain/the rest of WWII.  That would be pretty cool.