If you bought an item "off premises" – in other words not at the Seller's premises but online, or on the telephone, or even as a result of a visit to your own home – then you have 14 days to reject the goods from the moment that the contract is complete.
The conditions for triggering the 14 days "cooling off period" are that you have received the goods and also you have received various information referring to your cancellation rights.
http://www.legislation.gov.uk/uksi/2013/3134/contents/made
