Ferry Prices Hull to Zebrugge

Has anyone got any tips for this one.  I looked last year and they were approx £600 at this time (as they are now) but at the end of November the price dropped to £350 for a family of 4 plus van.  Stupidly i didnt book it there and then and within 2 hours it had gone back up.  Is there any good time or should i just bit the bullet and book now?  Many thanks