The unsolicited goods act covers this, you didn't buy the product, and have to return it if requested (not sure if you have to inform them about it). However it has to be returned at their expense. If not asked for them back they are yours after 6 months, however you are not allowed to use them and you need to keep reasonable care of them during that time. This is clearly not the case with the OP, as the retailer would like them back.