Simple question, is an off peak return Peterborough to Heathrow Underground valid on any of the following routes? 1 - Via Cambridge and in to Liverpool Street. 2 - Via Leicester and in to St Pancras. 3 - From Corby and in to St Pancras (I can get a lift to Corby) Kings Cross is closed on the particular weekend in September and I'm guessing there will be no trains south of Peterborough and I don't fancy Bertie the bus. Cheers!