Tube station ticket machines: a guide for visitors.

http://tidd.ly/1f69c02a Secret offers page at Ticketmaster
'Find out how to pre-order a Visitor Oyster card before arriving in London. And how to use our smarter ticket machines to top-up pay as you go credit on your Visitor Oyster card or Oyster card. Smarter ticket machines can now also carry out some transactions which could previously only be done at a ticket office.