For the benefit of others Plusnet agreed they advised me incorrectly about having to move supplier within 30 days (rather than just giving notice in 30 days), so have written off the termination charge.
Likelihood is many customer services staff don't understand the 30 day rule either. So that may offer an opportunity for anyone to leave outside the 30 days window if they give notice within 30 days, because I suspect the ombudsman won't treat the suppliers lightly if they gave out , as in my case, wrong advice.
This was their reply:
I have read through the notes on your account and the chat you had with Hannah.
I'm sorry to say that Hannah has advised you incorrectly regarding the price increase and when you can leave. If you wanted to leave without penalty due to the price increase you had to tell us within 30 days of receiving the email. This, you did, however you had to then leave within 14 days. The chance to leave didn't extend to whenever you were ready, there was a 30 day window to leave without penalty.
However, as Hannah has advised differently, in this instance we will allow you to leave without penalty. Your cancellation fees of £56.44 will be written off.