One problem related to radiator has been reported for the 2011 Mini Cooper S. The most recently reported issues are listed below.
Known issue with thermostat housing & sensor - Mini has acknowledged a known design issue that causes coolant to leak onto the thermostat sensor. Full replacement is recommended with updated design at customer's cost. Manufacturer should cover this cost since this is an engineering design defect and may lead to potential electrical fires. Affected parts are the thermostat housing, thermostat, thermostat sensor, upper radiator hose & crossover tube. Please refer to manufacturer's communication post: 10049421, 10046779, & 10045301.