Giving notice while still on probabation

Quick question that I need clarification on.

Joining a company and signing a contract to say that you will give 1 months notice if you decide to leave is fairly standard.
What if you decide to leave before your 6 months probabtion is up?

Are you obliged to still work the months notice, or can you give less.

I assumed once you signed the contract that you would have to work the month but I've been told by a colleague that by law, if you're leaving before the 6 months is up that you don't have to give a full month.
You can give whatever time frame you want...be it a week or 2 weeks.

Anybody know anything further?

Once you've signed the contract you are bound by its terms and conditions, so if nothing in it says that your notice period differs while on probation then you must comply with the notice period mentioned. Have you any holiday leave owing to you? You may be able to take that at the end of the month's notice, meaning you could be out of there in 2/3 weeks.