Has any work been done on the system recently? I had the cold water storage tank for topping up my hot water system raised recently. After that I was suffering from lack of hot water in a similar way to you. This turned out to be because the vent line from the hot water system had been immersed below the surface of water in the tank, in fact almost to the bottom. This caused a syphon to be set up when we turned on the hot taps, mixing cold water in.
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