First of all, consult your student handbook which should be on the Uni website, which should explain how classifications are given out. They do vary between unis, such as for mine, it is not decided at all on the average, but simply on the number of units of a certain classification, so if you get a borderline mark like 59 or 69, it's tough luck.
Also, your result only really bears the most weight in the first couple of jobs you do - after that, you work experience is far more significant, so being creative with job hunting and networking will get you far. Sometimes somebody with a lower grade who has more savvy and a sense for how companies work can do a lot better than somebody who is academically able but unable to apply their skills effectively, so it pays to have both types of skills.
Also, some firms have a section asking whether there were any extenuating circumstances that may have affected your grade - they certainly do for KPMG Tax and Auditing (where I have applied for, and have an interview for this week!) so an explanation can also be made there.
I know one person who dropped out of Imperial and didn't even get a degree, and now works in a decent investment banking position simply by working from the bottom up.
And just so you know, Carol Vorderman got a Third (albeit from Cambridge!)