You need to get this looked at urgently. Hot water getting into a cold water storage tank, especially if it's a plastic tank, can have catastrophic consequences.
Which header tank is it? There may be two. If so the smaller one is to keep the heating system topped up whilst the bigger one supplies the hot taps via the hot water cylinder. If it's the smaller one, it could be that the heating coil inside the hot water cylinder has failed, allowing the slightly higher head in the bigger tank to push hot water out of the cylinder into the primary water heating and central system then up into the small header tank. If that's the case the only answer is a new cylinder.
Another explanation for the tank overflowing might be a failed ball valve feeding the header tank but this wouldn't explain the fact that the water in the tank gets warm.
Finally, if the boiler thermostat has failed the water in the cylinder (or the heating system) could be overheating and expanding up through the vent pipe and into the top of the header tank. You can easily check for this if you can readily access the tank.
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