You don't need to do anything. You will just be billed the full published cost of the service you are receiving on a monthly contract.
What you could do is ring in and select the 'I am thinking of leaving' option and see what sort of a deal they will offer for a new contract. Alternatively you could just go elsewhere - the choice is yours.
Re: My Contract has expired, what happens next?
Usually with most companies nothing.
Your service will continue as it is. You may be able to get a better deal if you signed up for a new contract - if you wanted to (say cheaper rental, etc.) if you don't mind that then call CS and ask.
If you're happy with things the way they are, then nothing will change. Just that if you chose to leave, you can. Usually companies will want a notice period, don't know what PlusNet's is but companies like Sky need a month (don't ask me why).