I have IBT3, which is what is now know as CLAIT 3, I think. I took it about three years ago, and I think it's changed slightly in the last couple of years or so.
Since CLAIT 1 is very basic and you consider yourself to be computer literate, you shouldn't have a problem going straight in at level 2. It mainly involves using some of the more advanced features of the packages you learn at level 1. At level 3 you will combine all the packages with the addition of presentation and e-mail software.
You should be aware that the 'mistake allowance' is tightened as you progress through the levels. However, the minimum standard in my workplace is ECDL, which I have been told my IBT3 exceeds. I can certainly do far more than most of my colleagues who have the ECDL qualification.