Dual Flush Toilet - No markings to indicate make and model?

I have a dual flush toliet, which re-flushes once, approx 20 seconds after the initial flush. I've tried both the short flush and long flush buttons, which appear to work correctly, but in both cases there is a second flush. Have tried to take the lid off the cistern, and have tried to twist the ring on the lid which houses the 2 buttons to remove the lid, but it wont move, so not sure if this is the correct method of removal - any suggestions to both questions?