Complex sentence: Grimsby Town and Hull City were the only two professional teams which had official permission to play league football on Christmas Day because of the demands of the fish trade, but that tradition has now disappeared following the dramatic reduction of their trawler fleets in recent years.
Split into simpler sentences: Grimsby Town and Hull City were the only two professional teams which had official permission to play league football on Christmas Day because of the demands of the fish trade. That tradition has now disappeared following the dramatic reduction of their trawler fleets in recent years.