Sometimes my Spark has a hard time acquiring the required amount of satellites to set the home point. I then, on the the very odd occasion,  get impatient due to all the waiting and decide to take off without the home point being set. Straight away the drone goes into a tailspin and goes out of control. It’s very close to me when this happens and only 5 or 6 metres above the ground so I always retrieve it  but can anybody tell me why the drone’s navigational system seems to be totally off when you don’t set the home point? I thought setting the home point was solely  used for when you want to return the drone home.

Could it be the downlooking sensors of your Sparky are dirty?

I would:
A--clean the sensors
B--I usually send my spark up a few feet/2 meters off the ground to reduce interference, then I let it hover until it sets the home point
C--GPS is used for more than just RTH. It helps the drone stay out of restricted airspace, it is much more precise outside than using attitude and visual sensors.