It also occurs to me that charging for carrier bags is actually likely to have the opposite to the desired effect. In the past many people would use their carrier bags to get the goods home, unpack and gather up the bags before sending the whole lot for recycling. Now, as each bag is charged for, they'll reuse each bag until one-by-one they fall apart. Most people are likely to then simply dump the individual bags into the bin as it is too much bother to gather up bits of bags and hold onto them until they have enough to make a recycling trip worth the bother.
Also, with Tesco carriers already being biodegradable the idea of keeping them out of landfill because of the time it take for them to break down becomes a moot point. It would have made far more sense had the government merely insisted that all single use bags, from all shops, were of the biodegradable variety and allowed people to continue using them as they are used to doing.