When selling to consumers online or using distance-selling methods customers have the right to change their minds and ... return the goods. This period of time is known as the cooling-off period during which they have an unconditional right to cancel.
In the case of goods, the cooling-off period normally ends seven working days after the day the goods are received.
Consumers must inform (the seller) in writing - by letter, fax or email - of their decision to cancel.
Google "distance selling rules" for more info.
But I would certainly contact Heinnies ASAP and see if they will accept a return. I've always found them to be reasonable guys.