In message , at
20:40:28 on Mon, 14 Nov 2005, TKD remarked:
How is that different from having a valid ticket to (say) Elephant and Castle on the northern form
somewhere up north, and wanting to use prepay to extend the journey to Balham?

Because the entire journey will be on Oyster. If you started at an NR station you will have
to buy the ticket to Bank on paper.

The journey will no more be "all on oyster" than one which switches from
northern to DLR at Bank. Let's say they started at Cambridge with a
paper ticket to Z1, then changed onto the northern at KX and have got as
far as Elephant (or Clapham South if it's a ticket to Z12), and want to
continue to Balham by Oyster.
--
Roland Perry