Thanks for getting in touch @treesoh From what I can see we emailed you on 08-Oct-2019 at 00:01 from "email@example.com" with instructions on how to claim your cashback. As this is yet to be claimed, I've auto-redeemed this for you now which means you'll get a cheque within 30 days.
Re: No cashback email
The only email I received from Plusnet on the 8th is regarding an issue surrounding the router, which wasn't sent out on time. There's nothing either in my inbox, or junk folder regarding cashback. I checked for it specifically every day, so that's something you will want to look into lest it be said Plusnet don't want people claiming. I checked my Plusnet email account too and the emails from Plusnet are duplicated in there, but there's nothing about cashback, just the same ones which were also sent to my personal email. I can say with confidence that I never received such an email from Plusnet.
Perhaps the fiendish Saruman intercepted it? Maybe Sauron sent out the winged Nazgul to claim it for his own? As it is now solved, I can't say that I care too much, as long as we're clear that the email did not arrive.