How do I fix a compact dual flush cistern that overfills into pan?

So as you raise the float arm to its highest point the water is not shut off?

That suggests the cistern needs a new inlet valve - a job for a plumber very probably - shut off the water supply - remove the inlet valve and float assembly and fit a new one - turn water on again - adjust fill level.

Continuing Good Luck...
March 2017

Thanks for replying, much appreciated. I adjusted float and water fill level but water still flowing into cistern near float arm, apologies if that's not real name, but float is under water and water still flowing in even when float arm horizontal to cut off point. Cheers

Joyce
March 2017

The float adjusts the level of water allowed into the cistern.

Lower the float to shut the water off sooner and stop the overflow into the pan. If there's a locking screw on the adjustment be sure to do it up after you've made the adjustment. Set the fill level to half an inch below the overflow tube.

If this is incomprehensible to you (sorry) then operate the flush several times and observe what's happening.

There are videos on YouTube that might help.

There are different types of fill valve but the principle is the same - the water lifts a float and the float shuts off the water inlet.

Good Luck...
March 2017