Intermittent heater fan

[nigelattewell]

My 2016 508SW 180bhp estate has had an intermittent heater blower for the last couple of months. It has stopped for periods between a few hours and 3 weeks and returned for periods of an hour to 4 days. It either works fully (all speeds) or not at all and has never cut out or restarted during driving. If it works when the car is started, it will work for the duration of the trip. This seems like something sticking rather than a true random fault. Is there a relay for the fan and where is it? Any other ideas?