Assuming the water level in the cistern hasn't changed (you can tell by the water mark on the cistern) you need a new syphon kit - you should be able to get one fairly easily from a DIY store or bathroom centre. If the water level is up high, the ballvalve is letting by - it needs re-washering or replacing. Regards Colin Evans PBSL Group
That model of toilet doesn't have a syphon but rather a dump valve.
But anyway, the fault most likely is the rubber seal is faulty or has grit trapped in it.
It can be accessed without removing the whole valve. With the water supply off, the top of the inlet valve can be removed by turning 1/4 turn anticlockwise in the same way as a bayonet light bulb. Beneath is a black washer the size of a £2 coin. After taking note of which way up it fits, prise it out and clean/replace with new.
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