Insomnia is a very common and disabling symptom. Whilst evidence for the efficacy of cognitive behavioural therapy for people diagnosed with insomnia (CBT-I) is strong, few people seek help and not many services offer CBT-I. Less intensive adaptations of CBT-I have been shown to be valuable, and given the size of the problem and low rates of help-seeking, an accessible intervention with a large capacity is needed. Day-long CBT-I psycho-educational workshops (each for up to 30 people), to which members of the public with insomnia symptoms could self-refer, have been developed. This randomised controlled trial aimed to evaluate the effectiveness of these workshops. Baseline measures were taken from 151 participants, who were then randomised to experimental or waiting-list control groups. Scores of the experimental group and the control group were compared 3 months after baseline. Random effects models found a significant interaction between time and group, indicating differences between the control and experimental groups on the Insomnia Severity Index (ISI). Post hoc analyses indicated that ISI scores decreased significantly in the experimental group, but not in the control group. Promising results were also found on corroborative sleep diary measures. Access to the workshops was good, with 50% of participants having never previously sought help for sleep difficulties from their GP. CBT-I workshops proved to be both accessible and effective in reducing insomnia symptoms in the medium term. They may represent a feasible brief intervention with the potential to address unmet treatment needs of adults complaining of insomnia symptoms.