Hi all. I have booked some advance tickets from York to visit various spots in London in April. At the time I also bought a zone 1-4 Travelcard for ease of getting round the various stations I want to visit. However I plan a trip from Euston (on the fast line to get the locos in Wembley Yard) to Harrow & Wealdstone. However Ive just noticed that this station is one outside the boundary in zone 5. I understand that I can get an extension ticket ?. Do I buy this at Euston before travelling or on the train. Also any idea how much this will cost ? Thanks for you help in advance.