Level Sensor Leaking?

[BCH]

Last year I noticed coolant making its way onto the plug and rubber housing which connects to the expansion tank level sensor. The expansion tank had a slight leak so I replaced both the tank and sensor. After fitting I noticed the same leak up through the sensor.
I'd have to blow it dry every so often to stop the dash from flashing, plus I would have to top up the coolant level once and a while.

I have since replaced the the level sensor for a 2nd time and I'm still getting the same problem. Its not leaking around the thread, only where the actual plug connects. Have I just bought yet another leaky coolant sensor?