It's not the 5p, it's charge vs no charge. Consider two scenarios:
1) Bags cost 1p, but now cost 10p.
2) Bags were free, but now cost 1p.
I bet that the reduction in bag use would be far greater in scenario 2 than in scenario 1. It's like why a very cheap paid app is disproportionately harder to market than a free app.
1) Bags cost 1p, but now cost 10p.
2) Bags were free, but now cost 1p.
I bet that the reduction in bag use would be far greater in scenario 2 than in scenario 1. It's like why a very cheap paid app is disproportionately harder to market than a free app.