Legal Contradictions
Keep in mind that the original question was posted by a Brit and the responses have been US or from the Land of Oz ( Austrailia for the uninitiated)- completely different laws and obligations.
In the US - CP is contraband at the federal level - mere possession is chargable. In the original example, a data recovery specialist find CP in the process of recovering a drive. S/He is obligated by law to report - this is NOT a conscience thing - and failure to do so has its ramifications. Keep in mind that actually determining if a picture is CP or has been morphed is a factor. I would report and let those with the proper tools determine the authenticity.
The further disctiction between adult porn and obscenity is jurisdictional. Something legal in one baliwick may be chargable in another.
CD