Is my immersion heater overflowing?

Hi, I live in a flat and my external overflow pipe is dripping near enough constantly. We have checked inside the WC cistern and the level seems fine after flushing and before. We do not use the immersion heater and have not done for years but could this be the cause of the leak? It states 'combination cylinder' on it, if that helps?? If so, how can we fix it? Thanks very much in advance,
Jeni