Paid professionals should be able to organise their time correctly full stop. If like many events on the folk scene you have amateurs performing and timing is important then it needs policing. A firm but fair MC is vital. MCs are undervalued IMHO. They can make or break a good concert. Without making it obvious to the audience they have to police the programme effectively whilst at the same time keeping a calm jolly atmosphere going. Being a schizo sometimes helps.