Wow. What a pain! I had to register to this forum just to thank you all and add some info for the next victim... I mean IS customer to save some grief.

There are 2 types of Dual Push Button systems that lock the IS cover to the cistern, which require different methods to remove. And yes it is dumb that IS manufactured the toilet with the cover fastened, but apparently they fixed in later models.

1. Push the Big Button down, wiggle the little button until it comes off, remove big button, access screw that released the cover.

2. Then there is my situation, the silver ring that goes around the button is actually threaded to the button assembly. Once the ring is removed, the cover lifts off. My ring was STUCK and my two buttons do not depress independently, leading me to believe that the unit was broken or of yet a third type. Not so. I was able to free and unscrew my silver ring by putting on RUBBER GLOVES, gripping hard, and turning the silver ring anticlockwise while jerking the buttons themselves firmly but gently clockwise. This freed the stuck/jammed silver ring which was then easy to unscrew the rest of the way.

The reason I needed to get into the cistern in the first place was that the buttons had depressed and would not pop back up leaving the water running and the cistern not filling. I ultimately discovered this was caused by the "float" mechanism and come off its pinion, and became wedged under the flush assembly causing it to stick open.

I truly hope this helps someone in the future to avoid the frustration I experienced!

silver ring unthreaded and removed from button assembly. Top of toilet freely separates from cistern at this point.