Jay71, most of the two for one offers are rolled over twice a year, with the current crop due for renewal on Thursday. All you will need is that paper Travelcard bought from a rail station ticket office - no, there isn't one at Heathrow - and the vouchers either from this website or found in the back of leaflets about the offers you'll find at most mainline stations.

However. West Brompton, Kensington Olympia, Farringdon and Old Street won't sell the correct paper tickets as the booking office is shared with TfL, which sell tickets and Travelcards. Moorgate has a separate National Rail booking office, and both City Thameslink stations (Ludgate Hill and Cheapside) have ticket offices.