Complex sentence: When Gordon-Cumming arrived at the table, there were only two vacant seats; seating at either of them left him surrounded by the Wilson family, all of whom had been informed of Stanley and Levett's suspicions.
Split into simpler sentences: When Gordon-Cumming arrived at the table, there were only two vacant seats. At either of them, Gordon-Cumming would be surrounded by members of the Wilson family, all of whom had been informed of Stanley and Levett's suspicions.