Usually a Combination Boiler does not require any storage tanks. Instead it generates hot water 'on demand' by heating the cold water from the mains as needed.
The radiator system is heated by a hot water circuit which can be programmed. Combination Boilers heat only what you need. This is a far more space saving system than a Gravity fed one, no external tanks are needed, and provides endless hot water on demand so is particularly advantageous for shower users.
There is the drawback of slower flowing hot water particularly when using more than one outlet at the same time or filling a bath tub.