The outside air temperature sensor is located in the passenger side rear view mirror Brian...
As there are people on this forum with both LHD and RHD cars it is best to stick to LH and RH when talking about side of the car. These are always stated from sitting in the drivers seat facing forward. For the same reason offside and nearside should not be used. (it varies depending on which side of the road you drive) Passenger side can be either depending on where the driver sits! In this case it should be easy to tell anyway as if it is not on the one it must be the other, but I've just added this for completeness.
... it's that little black nipple that protrudes from the underside of the mirror housing.
They are known to go wrong and if I remember correctly they come as part of a bigger wiring harness if you try to get one from Renault but I have seen them (on e-bay I think) as a separate item so that you can splice the two wires into the existing harness without replacing the whole thing.
Correct. As Martin says, it comes as part of the (expensive) LH mirror loom, but you can get the sensor separately, remove the old one, and fit the new unit, joining the wires inside the back of the mirror head area.