It was one of those things that had been bugging me for a couple of years, but just not quite enough for me to snap and fix it straight away.
Well eventually I said to myself "How difficult can a stupid hatch latch mechanism be? This is annoying too much, I'm going to fix this straight away!". My hatch had been a pain in the bum for that long that it just seemed normal to have to jiggle the boot while wiggling the key to get the hatch to pop open. Sure the remote-unlock lever from inside the car still worked fine, but sometimes I was walking back to the car and just wanting to put stuff directly into the boot without going into the front.
In my car's case, the problem was extremely simple to fix (yay a quick win), however I couldn't find an explanation for why it ever occured in the first place.
I noticed that when turning the key, there was a considerable amount of distance that the key mechanism would move before it felt like anything was engaging. After removing the inner metal panel cover thing, I could see more clearly what was going on. It seemed that for some reason, the metal actuator rod, simply wasn't long enough - this gave it the distinct disadvantage of having to move the lock mechanism forward some distance before the mechanism would engage.
I couldn't decide whether someone had installed a new shorter actuator rod (unlikely), or if something had stretched or bent over the years making its length insufficient. This will always remain a mystery to me. Even though I do suspect the stretching theory, there was no evidence of anything looking like it had stretched or bent almost a whole centimeter.
I pulled the entire latch mechanism out of the car, and because I didn't have a longer piece of actuator rod type stuff handy at the time, I simply bent the metal arm that it connects to to give it a greater starting offset (see image below).
I added close to 1cm of offset just having a rough guess, and it turned out to be the perfect amount. FIXED and DONE. The car was back together in about 3 minutes and I was impressed by such a simple thing as my hatch lock mechanism responding beautifully to a simply key turn!