If it is the Carver/Truma 'crystal' water inlet then the non-return valve is in the stem where the jubilee clip can crush the stem and jam the valve. Make sure the clip is as far on as possible and that previous fitting has not made the stem oval.
Further if the heater is the Cascade then you have a non-return valve in the cold water inlet, if this is jammed open then as the water heats up the increased pressure will push hot water back into the cold feed.