Hello wynnema! You have received some good insights from others already, but I would like to add my perspective.
First, let me address the following portion of your post:
I was working on my roundhouse Kick last night experimenting with a few foot positions after watching the Power High Kick Video from Stadion.
While I believe everyone should "experiment" with their techniques to fine tune them, I wonder what input your instructor has on this issue more than what you get from a kicking video.
Next:
I have always struggled with the height of this kick but realised last night that I have probably been overturning my hips all these years...
Don't be too quick to condemn the 180 degree pivot, or think that you have been doing the kick wrong "all these years." There are different methods of executing every kick, depending on the circumstances. The roundhouse or "turning kick" (dollyeo chagi: 돌려 차기

, can be performed for speed, agility, and power.
Ok, now pay close attention to this one:
I think the way I have been taught is the traditional WTF TKD technique which is a powerful kick but unless you have an extraordinary range of motion in the hips, you will struggle to get this above shoulder height.
As an "old timer" I often get a kick out of the attachment some assign between techniques and organizations such as the WTF. "Traditional WTF" has virtually no meaning to those who learned all of these skills before the WTF, ITF, Kukkiwon, or anything else was created. These "traditional" methods of kicking do not come from the WTF. They come from generations of instructors of various Kwans who handed this knowledge down long ago. Most Taekwondo instructors (of any legitimate training) will teach to pivot further and turn the hips more for a level (horizontal) approach to a power kick. In more recent times, the diagonal sparring kick has become popular for faster, initial strikes with less telegraphing.
You mentioned "extraordinary range of motion in the hips." Most students can stretch enough to kick head high with a fully pivoted roundhouse (180 degrees), but if this kick is limited to chest high, or knee high due to a lack of flexibility then so be it. It is still a proper kick, and can be used in self defense. If your hips have limited agility, a 90 degree pivot is appropriate, and will free up the hip socket for more height, but might sacrifice some potential power (it will still do damage). I am 46 years old, and I pivot 180 degrees for my power roundhouse kicks, and I can get them over 6 feet high - but not everyone will work for that flexibility, or injuries might inhibit it.
Keep in mind, fully pivoted roundhouse kicks from the back leg are used primarily as finishing techniques, after your opponent has been dazed or injured. This is often not even close to what happens in tournament type sparring where most everything is a quick, stunning blow - - which resembles the type of
initial strike in the street. How many times does a tournament fighter assume a deep front stance and reverse punch? In the street, these types of stances and kicks are done at the end of the fight to finish off a stunned attacker.
On the other hand, if your using your front leg kick, a 180 degree pivot is very useful. Anyone who has done a seminar with "Superfoot" Bill Wallace knows that he throws three kicks from the front leg (left leg only): The roundhouse, side kick, and hook. All three are done with a full 180 degree pivot. Now, I know Bill has "extraordinary" flexibility, but there are many Taekwondoists who can kick head high with the full rotation. Perhaps a little more stretching will make it easier. Otherwise, it is perfectly ok to modify the rotation to get more height, if you need to, but 180 degrees is not wrong!
CM D.J. Eisenhart