I hold a paper travelcard for zones 1-4 (issued at a national rail
station), and occasionally need to travel to zone 5. Until last
December I always paid the difference in cash before the journey, but
the penal hike of cash fares meant that I have now bought a pay as you
go oyster. At the time of purchase (at Victoria TfL enquiry office) I
asked how I should use it for a such a zone extension - if I touched in
at the start of the journey I would presumably be charged for the Z1-4
section for which I already hold a travelcard. 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.

Well, yesterday I tried this for the first time, travelling to
Cockfosters. Staff there absolutely refused to deduct the correct
amount, saying I should put the whole journey on the oyster card. I
pointed out (politely) that I was following TfL advice, but that made
no difference. After about ten minutes arguing they let me through the
barrier (so I got a free ride), but this is clearly unsatisfactory.

How should I temporarily extend the range of a paper travelcard using
PAYG oyster?

Peter