The CEO’s comment that your period of maternity leave had been a ‘set back’ to achieving real change in your department is clearly discriminatory and this comment should not have been made. However, this comment on its own is unlikely to be sufficient for you to succeed with a claim for sex discrimination and/or constructive dismissal.
Furthermore, I understand that the CEO then stated to the person who was your maternity cover that your return had marked a return to ‘business as usual’ etc – i.e. positive comments in respect of your return to work.
Therefore, looking at the entire picture, I would not advise you at this stage to take any action. However, you should keep a written record of dates and details of any unfavourable comments made regarding your maternity leave and if further negative comments are made, you may potentially have a claim against your employer for sex discrimination and/or constructive dismissal.
In relation to the CEO’s comment regarding your period of maternity leave being a set back for the department, you could raise either an informal grievance or formal grievance in writing to your employer. However, this may blow matters out of proportion and you need to consider this.