Yet
once on the train
there is no way to validate the ticket on passing into Z5. The reply
was to tell staff at my destination station who would then deduct the
correct amount from the card.

That doesn't sound right. As I understand it, if you have a paper ticket
you are actually supposed to break your journey at the point where you start
using Pre-Pay. There used to be a statement to this effect on the
Oystercard website but when they merged it into the TfL site they seemed to
think 'Ask Oystercard' was all they really needed and I can't find it on
there.

G.