I will in a second, because it was the hon. Gentleman who started me off in discussing this misconception in the first place, so I will happily let him come back in.
A special offer such as “Buy one, get one free” on Persil is intended to encourage people to buy Persil in the hope that by the time the special offer ends, they will stick to that brand. It is a way of promoting a brand, and it has absolutely nothing to do with the supermarket.