Often with terriers, combining a 'leave' and 'recall' command has quicker results. Really condition the 'leave' well before using it with distractions then simply, when prey drive kicks in give the leave command just prior to the recall.

When I teach a 'leave' I train it so the dog looks at me before being rewarded. When conditioned well, the 'leave' before the recall cue has the effect of the dog stopping and looking to me so that the recall command is listened to - often dogs in high drive don't hear it as they have already started their run.