They are breaking the law if they have not sold that bag at the advertised higher price for a certain number of days in a certain period.
"Shops that display sale signs must follow rules - such as advertising the items at a higher price for 28 consecutive days in the previous six months in the same outlet. The original price must also be clearly displayed along with the sale price."
First step - report it to your local trading standards office - they will point you in the right direction.
"I was told that the goods I bought in a sale were �50 reduced to �35. However, I've discovered that they were actually �20 just before the sale began. Can the shop do this?
The shop may be committing a criminal offence under the Consumer Protection from Unfair Trading Practices Regulations 2008 by misleading customers into thinking that they are getting the goods at a sale price when they are not.
The shop should compare the sale price to the most recently available price. The product should have been sold at this higher price for 28 days before the item was sold at the 'sale' price. You should report this kind of sale to your local trading standards officers."
http://www.which.co.uk/advice/how-to-buy-in-sa les/frequently-asked-questions/index.jsp