I've received my Reward Card yesterday (Friday 9th Aug) and the letter instructs me to visit the website cited on the back of the card itself - www.plusnet.purecard.com - however that website appears not to exist. I haven't received the activation code either.
Dialling the number on the letter for help, there appears to be no humans to speak to, just a disembodied voice giving me options to press 1 or 2, neither of which deals with having a website that doesn't exist.
Fixed! Go to the fix.
@Chicken1973 This thread https://community.plus.net/t5/My-Order/PureCard-Activating-Prepaid-Card/m-p/1655805 reported the same problem. I believe from that thread the correct website is plusnet.purecard.com i.e minus the www
Re: Website for Reward Card does not exist
Cheers for getting in touch and I'm sorry to see you're having issues with activating your reward card.
Hopefully the solution @MisterW provided helps but if you need any further assistance feel free to yell.