If my flexible working application is disputed, should I go through my employer's own grievance procedures?

You should be able to use your employer's own grievance procedure (if they have one) if your application for flexible working becomes disputed. In this case, you should be entitled to be accompanied at grievance hearings by a union official from outside your workplace, should you wish. If using the grievance procedure, you should be very careful about time limits for any employment tribunal claims. This means that whilst the grievance is underway, in order to protect your position, you should consider lodging a claim within the three month general deadline from when the application was rejected on appeal or the breach of procedure occurred

In order to submit an employment tribunal claim based on your grievance, you must complete at least the first step of your employer’s grievance procedure and then wait 28 days. If you fail to do this first step, an employment tribunal may refuse to hear your claim.