I used to have it on various mondeos.
Can I ask a silly question, on the S max is the ESP switchable ? and if yes have you switched it on/off. If the light on the dash is permanent on then the system is off. Sorry if this is teaching oyu to suck eggs, but I've seen people on forums get this the wrong way round before.
It may not help to much with terminal understeer, but should be very quick to kick in when pulling out of road junctions with to much right foot.