Sorry to hear you're looking to leave, Sian. Please feel free to let me know if there's anything we could do to retain you as a customer.
We do usually require cancellations are made over the phone, however there's currently a wait of less than five minutes - it may be worth double checking you're calling the right number: either 500 from your Plusnet SIM, followed by option 2, or 0800 0791133 from any other phone, again selecting option 2.
Again, we'd love to change your mind if possible, so please drop me a Private Message if there's something I could do for you. If not, I wish you the best with your new Operator!