I've never encountered anything like this before... I left Plusnet on the 27th October and received my final bill (Refund for overpaid subscription charges) on 3rd November.
Today I was advised of a new direct debit payment and on logging in I see there is a bill consisting of advanced charges for the period from 17/11/2017 - 16/12/2017. It seems like a new account has been set up without my knowledge called 'Unlimited (Contracted without Phone)'. As I am receiving neither phone or broadband from Plusnet, I have no idea why this has been set up. I migrated phone and broadband in exactly the same way I have done successfully in the past. Any thoughts? Will be giving them a call this evening.
Edit: Just to confirm, when I navigate to my Home Phone Control Panel, I just get a message inviting me to switch my phone services to Plusnet and I don't see any evidence of their being a broadband service on my account either.