I'm trying to decide between a zone 1-2 and zone 1-3 travelcard. If I have a zone 1-2 card, can I travel to zone 3 or 4 and pay a little extra, or would I have to buy a ticket apart from my travel card for such a trip? If the former, how exactly does it work and how much extra do you have to pay?
Also, I notice that on the Tube Map, some stations that appear to be on the border between zones 2 and 3 are listed as zone 2/3 instead of one or the other (Clapham South for example). What is the policy on travel to those stations with a zone 1-2 card?