I understand this is an old thread but just in case someone else is having issues.....
I had the same issue, the link wasn't working - basically I did it on my top box. Use the APP on the actual top box and not your smart tv. Make sure you log out of Netflix, then close the app, then open the app up again and it 'should' work. It asked me whether I would like to link my account to my virgin account, then even asked me as I was previously on premium if I wanted the standard package provided by Virgin to be included and they would add it to my virgin bill for ease. Bingo, good luck 😉