Jonathan Johansson turned Ashley Williams in the box and his goal-bound effort was saved well by Hennessey, but his parry fell into the path of Roni Porokara who slotted into an empty net from 12 yards.
Saints 'keeper Howard might have done better at the goal, and with 20 minutes remaining he was nearly punished for leaving his box, but Imrie's goal-bound effort was cleared by Scott Cuthbert.