If a group session is missed (for example because of a school trip, staff sickness, etc.), try to fit it in on another day. If that is not possible, then deliver the missed group session instead of one of the individual sessions, but it is important for the children not to miss more than one or two individual sessions per topic. If individual sessions are missed, try to find additional times for the child to receive them.
Try to complete each topic on time to avoid the 20-week programme running behind, but to do this it is important to plan ahead, so that the point when a topic ends makes sense, i.e., the final topic session of the group narrative/story finishes at a point where the whole story is explored again.