No hot air and water sensor info

[Diagro]

Hey.

Two issues that need sorting on my 172.

First is the coolant sensor, I have a feeling it's broken as my fan doesn't come on too cool the engine but works with ac on.

Second is the heater. Just pushes cold air ;(

Any ideas where the sensor is so I can replace it?
Also what needs replacing to get hot air?

Sorry for all the questions of late, just trying to get the car in a state where it's good to drive and won't have any major issues on my drive to work and back lol

 

[GCC]

Is there definitely coolant in the system??

 

[Diagro]

100%.
It's at the max line

 

[Diagro]

Also when I stop after a run around dropping mates off the coolant is flowing but very slowly. Like a trickle

 

[Diagro]

I think it may be a thermostat stuck mostly closed? As top rad pipe is HOT HOT HOT. Actually most of the coolant pipes are Damn hot to touch.
It's around £55 for a new temp sensor and thermostat from renault. Would it be a good idea to replace them both?
Or do the cheaper ones from say amazon or ebay work just as well?