Allow students the time to talk. Check out this strategy. Honestly, everyone needs to be given a chance to talk. Put 30 adults together in a small room and tell them to be quiet. It is not a practical application of learning.
Talking is fundamental to how we learn and not allowing children that opportunity seems to me to be a little unfair. And quite frankly, it makes your job, as a relief teacher, that much harder.
The management issue is to keep the talk on the task at hand. Off task behaviour is not acceptable in a learning situation. All the more reason to teach that learning through talking is important.