False promises and advertising

Discovery are lying about their Healthy Gear cash-back facility. They are also guilty of false advertising. They are simply refusing to honour the advertised 15% that I qualify for on healthy gear items that I purchased. Instead, they are only willing to give me between 2% and 3%. I have been in contact with them and they simply refuse to give me my full cash back due to the way their systems calculate the amount, which is obviously incorrect. I don't understand how they can be so unreasonable. They obviously do not care about their customers. What an absolute ****! This has made me so angry that I will be laying a complaint with the advertising standards authority.