If its online you purchased the item it does not matter if its been opened or not ! You have 30 days to return products from online shops no question asked what so ever its the law!
That is not the law!

30 day returns, such as offered by Amazon, are their own policies going way beyond what is required by law.
Under the new Consumer Contracts Regulations 2013 you have the right to cancel your contract of sale within 14 days without giving any reason.
You are also liable for any diminished value of the goods resulting from the handling other than what is necessary to establish the nature, characteristics and functioning of the goods. So if they have been opened/used, the retailer is within their rights to make a deduction.
Also, if you paid for expedited shipping, and not any cheaper options that may have been offered the retailer only has to refund the basic shipping charge.
'Which' sum it all up clearly:
http://www.which.co.uk/consumer-rights/regulation/consumer-contracts-regulations
Copy/Paste:
Cancelling goods
Your right to cancel an order for goods starts the moment you place your order and ends 14 days from the day you receive your goods.
If your order consists of multiple goods, the 14 days runs from when you get the last of the batch.
This 14 day period is the time you have to decide whether to cancel, you then have a further 14 days to actually send the goods back.
Your right to a refund
You should get a refund within 14 days of either the trader getting the goods back, or you providing evidence of having returned the goods (for example, a proof of postage receipt from the post office), whichever is the sooner.
A deduction can be made if the value of the goods has been reduced as a result of you handling the goods more than was necessary.
The extent to which a customer can handle the goods is the same as it would be if you were assessing them in a shop.
Refunding the cost of delivery The trader has to refund the basic delivery cost of getting the goods to you in the first place, so if you opted for enhanced service eg guaranteed next day, it only has to refund the basic cost.
Exemptions
There are some circumstances where the Consumer Contracts Regulations won’t give you a right to cancel.
These include, CDs, DVDs or software if you've broken the seal on the wrapping, perishable items, tailor-made or personalised items.