I was aware of what I was originally signing up for... but I did not think\know that i was signing a new contract.
When I transferred from Plusnet ADSL to Fibre it was made clear at every stage that I was entering into a new 18-month contract so I don't know why that apparently didn't happen in your case. In fact I had assumed that meant that I would be liable for the full balance of 18 months' subscriptions if I chose to leave early, whereas in fact Plusnet charges a lot less than this.
Chris's reply is indicative of the level of service Plusnet normally provides, they will bend over backwards to help, even when subjected to abuse (which, incidentally, I am not accusing you of).
plusnet Extra 80/20 trial