replace the syphon(big bit inside) there cheap enough from any plumbing outlet. and not to hard to put in but dont foget to use silicone sealer on joints. or maybe adjust handle.
Have you got a syphon or a flapper valve? I doubt you have a syphon, as it generally will stop running, indeed if there is a fault, water won't come out. Maybe the overflow is piped into the flushong pipe, inwhich case it's the ballcock needs attention.
If you have a flapper valve, throw the damn thing out and put in a syphon, flapper valves are another modern gadget that doesn't improve on the old methods. They are expensive and notorious for leaking.
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