cold tank back filling.

[Bernie2]

How about the washing machine or dish washer? The ones with hot and cold supplies could have the potential to cross flow internally.

 

[tackleburger]

Turn off everything and cap off the hot water outlet at the cylinder both sides then turn on if that stops it you have cold mains connected to a hot supply somewhere. You can test this by opening the cap to the hot supply pipework or even better connect a pressure gauge to it instead of a cap end.