Handsome devil huh?
Clarification needed: During a British Action Phase I placed Leftenant Chard in the Building Barricade box and the 1/3 marker in the - 1 space.
Next turn I used my Action Phase to flip the marker to its 2/3 side. Leftenant Bromhead is on the table and available. His heroic ability says, "You may return this card to your hand to perform one additional Action at this time." May this additional action be used to complete the barricade?
Yes definitely, just remember that completing the barricade will leave Leftenant Chard exhausted, i.e. he's returned to your hand. So you couldn't complete a barricade and use a second action through Leftenant Bromhead's help, to then place the 1/3 done marker.