The Temperature Gauge In My Mk3 CUNA Fluctuates Between 50-70, Until It’s Warmed Up. Is This Just The Case Of Changing Coolant Temp Sensor, Or Is Their Something Else I Need To Do?

Many Thanks

Same here , owner of a 1.6 diesel 115 bhp for a month now. Water gauge doesn't have a steady climb to 90 degrees, it fluctuates but once it reaches 90 degrees it stays there. I also noticed that sometimes when engine is cold (after car sat for more than 8hours) after i drive 1 or 2 miles, i get a rapid ascend to 90degrees,  the water cooling fan comes on and stays on for the whole journey.It happened to my 3 times so far. Usually it takes around 5 miles of normal driving to reach 90. Car's been properly check and no errors been found. Interesting to see if others encounter the same problem.

No - they don't tend to rise steadily.  Especially an issue if you stop/start the car.  Unless the car is overheating or loosing coolant, you don't need to worry. 

Very weird , I worked as a driver at a large car rental company in the past , for a few years actually, drove thousands of cars but never seen a car water temperature going to 40 degrees then to 0 then 50 then 30 then 70 then 50 and so on. My start-stop function is off the second i turned the engine on. Been testing with both on and off and i get the same result.