If a customer has problems stalling, is it best for the instructor to continue to work on say, roundabouts, and then the more starting and stopping is done the better they will get at not stalling, or is it better to work on stalling for 5 to 10 minutes say and then get back to roundabouts?

This is something that bothers me, to score highly on the Standards Check you need to change the lesson to match the needs of the learner, but in real life these 'stalling' errors tend to sort themselves out and the learner can focus on improving roundabouts, for example and be more interested and engaged and therefore learn quicker.