Ian F. wrote:
Basic question I know, but if you hold a monthly Travelcard on Oyster
from zones 1-3 and you want to travel to zone 5, do you just add a few
PAYG quid to the card to cover the additional fare?

Thanks,

Ian

That should do it.

You can also just touch out, and the amount will be debited to your
card. But you won't be able to re-enter the system until you have paid
the card's debt.