There are 2 or 3 Threads running in these forums about lack of payment of cash back and I too have commented in them, sometimes Plusnet Staff have commented but at this point in time there seems to be a silence come on.
I'm very disappointed my cash back is outstanding.
Was thinking about other means than these forums about contacting Plusnet but your experience as put me off as that too seems a waste of time.

One benchmark of the system working ok:  28/2 Order; 10/3 BB start;
11/3 Cashback request made;  23/3 Phone start;  22/4 £50 cheque, dated 15/4, received.

Approx. an hour ago Royal Mail delivered Plusnet letter and cash back cheque, which is now on its way to my bank.