You unscrew it from under the cistern, this will require removing the cistern, if it is close coupled, attached directly to the toilet, there are two butterfly nuts holding the cistern in place, just isolate and disconnect the water supply, undo the two butterfly nuts and unscrew it from the wall, you will then see the securing nut underneath.
Tim Carr
March 2008
Briefly - turn water off, disconnect water supply pipe and overflow (if there is one). Flush loo, disconnect 2 wingnuts that hold the cistern to the loo pan, lift cistern clear and empty. The syphon is held in by the large plastic nut that you can now see, which probably also holds the cistern clamp in place. Disconnect wire from flush handle, unscrew plastic nut and the syphon pulls clear. Replace syphon, apply silicone sealant to any faces that are under water, replace the large doughnut washer that sits between the cistern and the pan (add more silicone here) and put it all back together again.
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