Alexander rushed back to the line in vain but aggravated his right knee in the process and he was carried off to be replaced by Rubeun Rabstein.
A minute after the restart, Butlin was clean through and slid the ball beyond Rabstein but off the keeper’s left hand post.
Butlin then had a shot saved and then Langston had a shot cleared off the line by Rob Norris.
Dynamo midfielder Curtis Burrows almost levelled, forcing a double save from Curtis Pond before Jono Atherton should have added a second but put the ball inches wide of the target.
Just short of the hour, Atherton drew a penalty after sprinting clear from midfield and Langston converted from the spot - his 13th goal of the campaign.
Butlin seemingly did everything but score and headed straight at Rabstein from 6 yards following a Dilano Reid right wing cross.
Finally Butlin got his goal scoring an absolute piledriver in stoppage time.