In addition, a nested study examining the mediators and moderators for change and maintenance of change is included. Specific and general mechanisms of change will be explored to include positive beliefs about repetitive negative thinking, therapeutic skills usage and therapist behaviours. General mediators including the therapeutic alliance and participant's expectancy / credibility and will be examined. The study protocol, information on the study, informed consent will be submitted for approval by the NHS England Research Ethics Committee and Health Research Authority approval for the study will be sought.