P&O ferries, how to get cheapest tickets?

[IKnowAMouse]

We are going to use the Hull / Rotterdam route next summer but as ferry virgins haven't a clue about the tricks for booking cheap tickets. So things like is it cheapest to book early (like the budget airlines)? Are there any deals you can get (like Tesco clubcard ones)? Is it cheapest to book direct?

I feel very helpless not knowing, thanks

I am quite interested in this. The route is not out yet. But from previous years, they start quite cheap and then daily will increase. Although I don't know when 2015 will be released.
As far as I know, in years gone by you could use Tesco clubcard vouchers but not any more. Although you can use them on the Channel Tunnel.