Combi Boiler - hot water from cold taps

[Acorn764]

We have realised this morning that hot water is coming from the cold tap and is filling our toilet cistern in our upstairs bathroom (not downstairs). What would cause this and is it a simple fix? Seems like a thermostat or switch gone wrong but I am clueless about plumbing!

I think @PigletJohn is needed!

I think you probably have a faulty mixer tap, most likely one with ceramic discs and probably a joystick, which enables water to mix inside, and migrate down the pipework. Might also be a shower mixer, especially a bar type.

If you feel the temperature of the pipes under each mixer tap, you will probably find it.

Just to check, you have a combi boiler, you do not have a cylinder, nor any loft tanks, and you are not going misled by pipes getting hot on sunny summer days? Are you aware of any pressure reducing valves in your home?

Not a mixer tap, it's two separate hot and cold taps. Plumber is coming tomorrow but problem has stopped now weirdly, so just one freak morning with hot water in toilet cistern and out of cold tap. We did have the water company in last week to look at fitting a meter, we are in a flat with a complicated supply apparently. And yes definitely no water tanks anywhere, just a combi boiler in the kitchen.