Yes, an ion exchange water softener will work with a combi-boiler and reduce the scale present in your existing system.
Naturally soft water dissolves chalk and lime as it travels underground, creating hard water. Once softened, it is once again capable of dissolving the chalk and lime in your pipework and hot water cylinder.
Heat plays a role in speeding up the process so the scale will be removed from the hottest surfaces first, such as the heat exchanger. This will improve the performance of the heater immensely and quickly restore it to its original efficiency.