From the website:
Who do I contact if I want to return my product?
If you have received your product and then subsequently want to return it, first please find your order in the ‘My Wowchers’ section of your account which you can access here:
https://secure.wowcher.co.uk/myaccount/vouchers
Just below the order you will see a Need Help link. Please click on this link and follow the prompts to request a return of your item. If there is no Need Help link then you will need to contact the merchant directly. Their contact details can be found on the Wowcher itself.
If you still require assistance after following the above steps, , please contact us here:
https://secure.wowcher.co.uk/myaccount/csform