I buy all my groceries,petrol (and use other departments) from Tesco using my Amex premium plus card. What with that and other purchases I received my free companion ticket 9 months into the year. ( Off to Rio next April!) For the remaining 3 months any spend on the card accumulated miles but I had to wait until the next card year to start saving towards the companion ticket. As others have said, you have to fly out by the end date of the redemption voucher.
A far better way to use tesco vouchers than frittering them away instore!