Where to get the best ferry prices?

We're thinking of going to Switzerland next June and this afternoon I was looking into ferry prices, but at present I'm completely bamboozled.

I haven't decided yet whether to go by Newcastle - Amsterdam, Hull - Zeebrugge, Dover - Calais, or Eurotunnel so I was looking at all of these. Obviously the short cross-channel crossing is always going to be the cheapest, but we've done that before and the long haul from Perth to Dover is a bit of a pain.

What I've found so far is that there is a huge variation in prices on all the routes depending on what website you're looking at, so I'm looking for advice as to where is the best place to get the best deals. Am I likely to get a better price by booking through the Caravan Club or the C&CC? Is it better to book early or wait until near the time?

Any advice that more experienced continental travellers can offer would be appreciated.