 After that candidate was quickly reassigned upwards, Leahy returned to Tesco in 1979 as a marketing executive.Tesco was a resolute market follower of the two leading brands, Marks & Spencer as the then world's most profitable retailer, and Sainsbury's as the world's most profitable food retailer.Leahy concluded that Tesco should stop following a strategy of catch-up and start leading through market knowledge, which led to his success in devising and implementing the Tesco Clubcard loyalty programme and also monitoring the shopping habits of Clubcard holders.