Hi Simon,
Your only recourse is to replace it. To do so you will need to remove the bottom half of the dash to access the connector. You will also need to remove the dash surround and the securing bolt that holds it in place.
The photo shows the sensor, the fan is behind that. You can actually remove the whole thing from the front though you will only get around 6 inches from the housing. You may then be able to blow air back through the fan.
If that doesn’t cure it then replace it. I think they are available for around £40.
This picture shows how far it comes out of the dash.