So your back to an mixer or shower valve letting by and back feeding into the hot.
Check mixer tap/valve with mains cold, including the washing machine.
An easy way to check this is to run the hot water until the pipe is hot, then keeping your hand on the hot pipe open the cold tap, does the hot go cold.
After drawing off the water from cws tank did/has the water level risen at a noticeable rate, I find that generally if it's a slow, overnight before it drips scenario, it tends to be coil in cylinder, a fairly quick rise, 30mins or so, it's a mixer of some sort.
Isolate the header tank in loft, and see if the level of that tank drops, if it does the chances are the coil's gone.
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