Had this
problem myself, the button is still broken but I did manage to lower it into the horizontal position.... at least I can use it now and it doesn't rattle anymore!!!
If you press down the release button, you'll see two holes, one either side of it. Get a torch and shine it in one of them and you'll see a small metal bar, it's basically the release button. Use a screwdriver to push it downwards (downwards when the armrest is upright anyway).
Sounds easy.... it is if you've three hands because you need to press the switch at both sides at the same time, probably while still holding the torch you needed to shine into the second hole to see the switch.... you get the picture anyway!
I managed to get mine down to the first stop, but it's still too high so I'll have to give it another go this weekend and try to get it lower.
edit - incidentally, the rattle disappears because the button is no longer free to move up/down in motion with bumpy roads, it can only move front to back when the armrest is horizontal.