Our defensive plan against a target man is basically 'hope he heads it badly'

I’d organise the second ball before worrying too much about stopping every first header. Against a strong target player, your centre-back may not win every duel. They can still make the contact difficult, stop the clean flick and influence where the ball lands. Then your midfield need to be underneath the contest. If both centre-backs challenge the same target and the midfield are 15 yards away, the opponent will collect the knock-down all day. I’d normally have one defender competing, one covering the space behind, and the nearest central midfielders squeezing around the likely landing area. Full-backs also narrow because the second ball often drops into the channels around the duel. Train with realistic long passes into the target and score the defending team for securing the next possession, not only winning the header. The target man is only half the problem. The dangerous action is often the teammate who knows where the next ball is going.