From number 3 of the cashback faq :https://www.plus.net/home-broadband/faqs/cashback-and-vouchers/#how-to-claim
If you've signed up to a cashback or Multi-Choice voucher offer, you can claim your Plusnet Reward in just a few easy steps:
You'll get an email sent to the address you provided when you signed up. You'll normally receive your Plusnet Reward email within 10 working days of your first service being up and running.
Open the email and click on the link - it's valid for two months from the date we sent it. You'll then be directed to an online form. Fill in your details into this form and then click 'submit'.
You'll then see a confirmation screen - your Plusnet Reward is on its way!
You should receive your reward within 30 days of receiving confirmation it's on its way. It will be sent either via post or email, depending on the type of reward.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
