As I understand it
She was a recipient of a whatsapp message sent to a group which one of the other recipients reported to the Police
Police on investigating found that she had received it along with others
It was found that the clip had been forwarded to the group by William's sister, who had recieved it from her partner. Williams was one of the recipients.
WhatApp, by default, downloads a copy of any sent media to the recipients phone.
A recipient is therefore 'in possession' of any media sent to them.
WhatsApp records showed Williams had the app open and the thumbnail of the video visible on her screen (and therefore 'available to look at') for 12 seconds.
Williams, her sister and her sister's partner were arrested at different times, but were all on trial together. The actions of Williams following that clip being received, following her sisters arrest, and the claimed reason Williams was sent the clip, were all explored at trial. Evidence included call records and records of meetings between defendants before arrest.
There are three statutory defences to the offence:
a) Legitimate reason for having the photograph in their possession
(b) Not seen the photograph and did not know, nor had any cause to suspect, it to be indecent
(c)that the photograph was sent to them without any prior request made by them or on their behalf and that they did not keep it for an unreasonable time.
She went with defence B at her trial I believe, the defence fell apart when evidence of the text messages and meetings between her and her sister were presented to the court
Both of the others she appeared with accepted they had possession of the indecent image and she couldn’t rely on their support
she still remains convicted of possessing indecent images of a child