I know this is a popular topic but I have recently joined and signed up for Unlimited Broadband, prepaid for the full year on 11th February, and finally went live 27-28th February 2019. I am expecting an email to come in soon about how to claim the £75 cash back as offered by your website at the time I signed up (it said the offer expires on 13th February so I did sign up in time) - but there is no email, and no information on My Account on how to go about this.
I'm sorry you haven't received your cashback. I've had a look at the offer code on your account and it looks like you signed up through an affiliate site or link that offered the £75 cash back and is valid.
As you haven't received it I've had to send it to our billing team to check with the affiliate and have raised this ticket on your account. I'll be back in touch once I've got a response.