dual flush cistern toilet

[zingo1327]

I've had a dual flush cistern fitted but there doesn't seem to be enough water to flush the toilet. The guy said it was because I am using an overhead tank and not direct mains water.I had a look inside and the water level is only about a third of the way up the cistern.
Is what he says true or can it be adjusted to give a higher water level and in my eyes a better volume of flushing water.
thank you in advance

[Tridymite]

The source of the water is irrelevant. If the level of water in the cistern is only one third then that is the cause of the problem. At the pivot end of the float arm you will find and adjustment screw. First slacken the locking nut then turn the screw anticlockwise by several rotations. When you release the arm more water will flow into the cistern. Progressively adjust the screw until the water level is approx 15mm below the overflow pipe. Don't forget to re-tighten the locking screw.

[buildersmate]

However fitted it for you and then tried to excuse his incompetence with the explanation doesn't understand how such things work.
You could try and get him back to put it right, or attempt it yourself using Tridymite's clear instructions. Some dual flush floats work a little differently to that described above - but the principle is always the same - you need to shift the float such that it is in higher position before it cuts the water supply.
You could PROVE to yourself that this is the problem (before attemting adjustment) by holding the float down as it refills, allowing the water level to rise higher. On flushing you should then get a decent flush volume.

[wildwood]

You must have a different system to mine. The overhead tank only fills the warm water cylinder. The cold supply, such as for the cistern, is filled via the HP mains.