Can anyone let me know whether the Hero 5 camera is good and able to be used for tennis.
for example could I clip it onto myself whilst playing to record my strokes or would it simply fall of with impact of stroke and running around?
Does the Hero 5 have the ability to mount on a tripod (i.e. a 1/4 x 20 receptacle). If so, use it on a tripod off to the side, or use a camera mount that goes on the top of the fence behind the player. You can learn a lot from that angle...just as much as from the side in my opinion, having done quite a few of those videos. From what I've seen of putting it on your own body, that's too confusing and doesn't really give you any good data. Might be fun though!
I use Hero 4, they are really good cameras, particularly when you shoot at 120 fps. A tripod is the best solution and easy to use with the Hero cameras. The only downside is the Heros are on the expensive side just for recording tennis, but if you can use it for other purposes, then it is a good choice.
A tablet mounted on a tripod is great as you have a large screen to view during a practice session. However it sounds like you are not looking for a coaching aid rather than a fun view point. There are numerous GoPro clips on YouTube showing this effect, they are pretty tedious to view, I don't think it's something you would use more than once ie strapped to your forehead.