Will not honour 7 day return policy

I bought a FloorMax for my Mom on 27th Dec as a present. When she tried it out, it did not do what it claims to do. That is to pick up dirt from the floor. We cleaned it and put it back together and tried to return it with the packaging and cash slip. We were told that because it was opened we could not get a refund only swop it for something else. We did not want anything else and asked for our cash back. The shop assistant was adamant that we could not have this option. It states no-where on their slip that you can't return in the 7 days if the item has been opened. Obviously it was opened to try the product. I have emailed their customer care and still no answer. According to the National Consumer Act, you may return an item if it is not as advertised. Will never buy from there again.