Sorry, dumb question probably, but I find it difficult to really understand the fine points of Oyster card-plus-Travelcard: I live in Germany, already have an Oyster card from my last (lovely) holidays in London and should like to add a 7-day-off-peak Travelcard for Zones 1 and 2--- is that at all possible? And if so, how should I go about it? Can this be effectuated from abroad? Thank you, and kind regards, Magdalene