read this here consumer direct
they say
The Consumer Protection (Distance Selling) Regulations 2000 state that unsolicited goods which have not been ordered and are sent ‘out of the blue’ can be treated as an unconditional gift; in other words, you can keep them or dispose of them as you wish. The Regulations make it a criminal offence to demand payment or threaten legal action to obtain payment for unsolicited goods or services.
However, it is advisable that you write to the trader, obtaining proof of postage, stating that you have received an unsolicited item that you are aware that you are entitled to keep this item as a gift, but that you are giving the trader the opportunity to arrange collection within 14 days at his/her expense. Although you do not have to do this, we recommend it in order to avoid future attempts to secure payment.
See the bold bit, if you want to keep you can.