My money is on a new sign up promotion.
Get your nan to sign up and get £20.
However, in the small print it will clearly define how to get the sign up bonus only for the t&c to clearly define the opposite.
You have to raise an individual thread on the message board to explain the 2 clearly defined ways. Be called stupid. Raise a customer service ticket to be told one thing and then keep an eye on Twitter for another.
I can’t think it can be anything else.