Have to read the full study, but longitudinal surveys tend to be good quality as they are bloody expensive! Loads of people get lost to followup etc.
Take away point that flexitime doesnt reduce stress as much as reducing working hours is a good one to bear in mind.
The cynic in me would say that public sector/flexi time roles are predominately done by women, and with the austerity agenda means these are not low stress coasting roles! Need to look at how they sampled.