Welcome to the forums.
From https://www.plus.net/home-broadband/ in the legal bit
Unlimited Fibre Broadband from £5.00 a month for 18 months then from £14.99 a month when you take line rental– 18 month contract:
- This price is available to new Plusnet customers signing up at www.plus.net or by calling Plusnet between 14th September and 4th October 2016.
So only new customers get the best deals, existing customers don't.
You could try discussing new contracts/offers with the Customer Options Team on 0800 013 2632 (Selecting the option for thinking of leaving) to see if there's anything they can offer you? Of course, there is no guarantee that they will offer you this deal but if you don't ask them you will never know.
Any staff posting on the forums cannot discuss deals so you won't get an answer here.
Ex - Plusnet Customer (2009 - 2023) now with BT