how do I get rid of

Hogweed seems to be everywhere in my garden, I always remove flower heads before they can seed but each year there seem to be more and more throughout my garden, which is quite large. I have tried digging them out but they have such long roots I rarely succeed in getting them out completely then of course  they just come back! I have tried spotting them on their leaves with a systemic weed killer and I cut them back to the ground when I spy them. Anybody got any solutions?.