In reply to DaveHK:
Always book in advance, some companies charge more if you book on the phone rather than internet and often it has to be at least 24 hours in advance. Your friend may have been referring to the situation many years ago before phone and internet booking was common - back then you could just turn up, choose the next ferry and buy your ticket, but that was a long time ago.
Apart from the price at certain times of the year ferries can be full up - I just got an email from Eurotunnel warning me that for Easter some days were nearly full already for the "frequent traveller" ticket I use (10 trips to be used over 12 months). Nowadays with internet you can check out the few companies concerned quite easily.
If you do book you can usually go earlier or later without paying more unless they are full or the change puts you in a more expensive crossing - they all seem to have about three different prices. It's well worth planning in advance, which is a pain but it's the way the world has gone, like for trains and planes spontaneity seems to be out of fashion.
PS. I worked out the other day that I must have crossed the channel for getting on for 400 times now and it changes fairly often so it's always worth checking rather than assume that a company that was cheap still is, or exists even!