From the CPS,
"Charging ‘Possession’ or Charging ‘Making’?
Prosecutors must bear in mind what needs to be proved in respect of possession of the images. Much will depend on (a) the location of the images on the device (b) how they came to be located there and (c) how accessible/viewable they are in that location without specialist knowledge or software. By way of example:
A person who views an image on a device, which is then automatically cached onto its memory, would not be in possession of that image unless it can be proved that he / she knew of the cache.A person who downloads an image from the internet and then deletes it such that it is ultimately recovered in the unallocated space or clusters will not be in possession of that image unless it can be proved that he / she has the wherewithal to retrieve it.
The case of R v Porter [2006] 1 WLR 2633 supports the view that, in normal circumstances, deleting images held on a computer is sufficient to divest oneself of possession of them. An exception would be where a person is shown to have intended to remain in control of an image even though he has deleted it - that will entail him having the capacity (through skill or software) to retrieve the image. Where images have been deleted prosecutors may wish to consider whether they can charge the suspect with possession of an indecent / prohibited image on a date between either the purchase of the computer (or reformatting) of the hard drive and the date that the computer was seized.
In each example, the person would however have "made" the image in question. Subject to there being evidence of the act which constituted the making and the necessary mental element, an offence contrary to section 1 of the PCA 1978 is preferable and in most cases would suffice.
The use of section 160 of the CJA 1988 is becoming increasingly rare. It is better reserved to cases where the evidence is unambiguous in showing genuine possession, for example, where a suspect has the images in printed form or has saved them into a clearly labelled file within the hard drive of the device. The charge of 'making' also has the advantage of being widely interpreted to cover such activities as opening attachments to emails and downloading or simply viewing images on the internet. By contrast, the same conduct often cannot lead to a possession charge."