At the risk of providing ever too much information, a useful resource is the progression maps provided by the Govt. For each attainment target (on this they call "Statistics" "Handling data and probability") there is a progression map which breaks each AT down into a series of skills, which by name don't quite match the learning objectives or the APP criteria in other documents. Each of these skills within an AT has up to 10 steps (not NC levels though) in progression of understanding and ability. In each step you can click "View Step" for further details of what the step means, a link to the supplement of examples from the old Framework folder, a section of probing questions and a section of what to do if the students say "I don't get it". All of this is found at:
http://nationalstrategies.standards.dcsf.gov.uk/secondary/intervention/progressionmaps