My Roca toilet has a leak under neath?

My roca toilet has a leak underneath the tank, it seems to have left limescale from the water, around the washers of the fixing bolts, further more more the cisterns seems to re-regulate itself all the time, and there is water trickling in to the bowl, which i asumme is the overflow system working properly. How do I fix this, will i need to replace the bolts and the rubber rings? and how do I removed the top (crescent moon + round flush mechanism/two flush)?