Has anybody ever taken a jazz exam and (i'll probably sound a bit thick) what is the difference between them and standard exams?

No, I've not taken a jazz exam but if you compare the syllabus for, say, Grade 1 Piano and Grade 1 Jazz Piano (or any instrument for that matter where both options are available) you will see that there are some similarities in the overall structure of the exams.

However, the scale requirements are very different and, in the jazz exams, there is a considerable emphasis (even in the aural and quick study) on improvisation, recognising appropriate "grooves" etc.

If you want to learn more, pick up some of the published material and have a look at it for the jazz exams and then you will begin to appreciate the differences.