I’ve taken MX5 to Twickenham Mazda dealer this morning - they’re going to drain the clutch fluid and think the loose pedal travel was due to old fluid (losing its viscosity) maybe combined with the heat of stop start with heavily loaded car. £75 for 1/2 hr labour (it’ll take longer but as I just had it in for full service and MOT they’re charging 1/2 hour). They said they will have checked the Slave Cylinder for leaks in the Service but will have another look.
Will report back.