Originally Posted by stargold
AFAIK That won't work for travelling using Heathrow Express during the disruption weekends - having a Z1-6 Travelcard is different from having a Z1-2 + Prepay balance.
You can only use your Prepay to top up the balance if you are making a tube journey (or selected rail journeys). For other National Rail journeys (and generally where there are no Oyster readers) you have to buy the appropriate ticket/extension in advance of travelling.
Sorry, you mention that it will not work on the Heathrow Express... I did not plan on taking the Heathrow Express... I was talking about taking the Picadilly line between Heathrow and central london. If I have a Z1-2 travel card on Oyster and I travel outside Z1-2 (with prepaid on my oyster card)... the additional charge will be only for outside Z1-2???