Two month cards at a set price (perhaps with a small discount versus two one month cards) aren't available; I don't think I've ever seen anyone buy or use an odd period Travelcard. Seven day and longer season tickets can be bought up to 30 days in advance of the first day of validity. Season tickets can be piggy-backed on one's Oyster card as well so the Travelcard's validity will end and the next begin without a hiccough.

corky, Eurostar tickets are a separate entity from TfL products. If you've not booked yet please do so soon as fares rise nearer the date of travel.

For a five day stay in London seven day Travelcards may be the business. At £27.60 you and your niece will have unlimited use of the Underground, DLR, Overground and National Rail services in zones 1 and 2 and bus use in all six zones.

Buy the requisite seven day paper Travelcards at the First Capital Connect ticket office at St Pancras; they'll be sold to you on National Rail ticket stock and can be used with the two for one offers.

When you register at the website and print vouchers you'll be asked to nominate a station; you'll see that no central London stations are listed as the offers are for those travelling to the capital by rail. Choose any station you wish when registering though, as you won't be asked for proof of travel from that station when you use the offers.

lindalah, if three are travelling I'd say yes, buy Travelcards on Oyster. Easier to use than paper tickets, add a bit of PAYG pre-pay and you'll be able to travel outside the paid for zones with ease.