It's very awkward for the CPS not calling SW to testify. It's the elephant in the courtroom. She apparently was satisfied that Lucy was lying about SN, she went through an investigation process with social services, and as her mother would be the one who would care the most about Lucy and be most watchful for any substantiation to what Lucy was telling friends and vigilant from thereon in to what was happening under her roof. She would make an excellent witness for the defence.
Social Services should also be called to give evidence, since they were also satisfied that nothing was going on in the house so I would expect them to have visited the home and assessed the risk the school pointed out, including Lucy being looked after by a group of men who were barely adults.