Advice is given on the website of Graham Dilloway, a computer expert witness.
The most useful pages might be:
Regarding definitions:
http://www.dilloway.co.uk/definition-of-indecent-image-offences.html
Issues around whether there is specific evidence of a deliberate act:
http://www.dilloway.co.uk/appeal-judgement-and-evidence.html
Misunderstanding by officers of what has been found:http://www.dilloway.co.uk/police-interviews.html