The combination boiler has taken the market by storm, very efficient and compact.
Solar hot water can be stored in a single coil copper unvented cylinder, and fed to the combination boiler but precautions would have to be met.
Boilers were not designed to accept hot water supplied to them (instead of cold), and because of this only a few manufacturers would honour the warranty (check with the individual boiler manufacturer). After the warranty period of course then solar may be added.
A solar mixing valve would need to be fitted on the cylinder to control the water from exceeding 55 deg c
The hot outlet from the cylinder is then piped from a solar mixing valve to a 3 port valve. If the heated water is above 55 deg c, then the hot water will go directly to the hot taps, by-passing the boiler. If below 50 deg c, then the heated water will be taken direct to the gas boiler (cold in). When a tap is opened the boiler would initially fire up but the thermostat or thermistor in the boiler would sense this incoming hot water and the boiler would then extinguish the flame completely because the water is already hot enough, or it would simply top up to the required temperature as set by the boiler thermostat, IN THIS WAY IT IS ALWAYS SAVING ENERGY.