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How can I stop it overflowing into the bowl?Villeroy Boch as below?

Water overflows into the bowl,not a lot,and it sometimes doesn't overflow.I have tried adjusting but not successful.
The Cistern D/F OMNIA PRO 77191101 WH PAT.Villeroy Boch.This is the make and fitted in 2009.
How can I fix it?
Richard G., March 2013