Complex sentence: The circuits are grouped in thirty-one districts covering Great Britain, the Isle of Man and the Channel Islands, each supervised by a district synod and a district chair, except the large London District which has three chairs.
Split into simpler sentences: The circuits are grouped in thirty-one districts covering the whole of Great Britain, the Isle of Man and the Channel Islands. Each district is supervised by a synod and a chair--except the large London District which has three chairs.