Sounds odd. The header tank pressurizes the hot tank by feeding it from the bottom. If the water in the hot tank becomes excessively hot due to a failed immersion thermostat causing the element to run continuously, water could boil in the tank and eventually some of it could be expelled through the expansion pipe into the tank. The same would happen if the tank is heated by a back boiler and the circulation pump fails, or radiators are turned off upstairs, causing the tank to overheat. I've noticed in my own house that warm water in the small header tank that tops up the heating system can become warm due to convection. However, as far as I understand it, for a vented heating system, there should always be two header tanks. Otherwise anti-corrosive chemicals from the heating system could end up in the water feeding the hot taps, and cold taps of hand basins (this cold water isn't supposed to be drunk because it came from the header tank and can have all sorts of dust and dirt in it!). In any case, hot water expansion shouldn't cause an overflow unless the ballcock is faulty or it's set in such a way that the water level in the tank ends up being too close to the level of the overflow.