Pumped water will always take the easiest route, on the older system it is pumped from the boiler to the airing cupboard to 2 zone valves one opens and lets it circulate through the radiators, when the hot water cylinder thermostat calls for heat it opens the other zone valve which lets the water circulate through the heating coil of the hot water cylinder heating the water, because the cylinder is a short route it returns back to the boiler, so it circulates much easier through this coil so slows down the circulation through the radiators which is normally the furthest ones away, Normally fitted is a valve inline to the cylinder heating coil to restrict the flow so as to keep a good flow through the radiators by forcing more water through them.
If it has this valve then turn the heating off, call for hot water so as to circulate the hot water through the cylinder start closing the valve until you hear the pump change note this shows that you are restricting the flow through the cylinder then simply open it a touch until the noise goes, this shows that it is at the point of being slightly choked giving more flow to the heating when that comes back on, That's it.
Hope it helps.
Mike.