Complex sentence: The Prince of Wales, who had known Lady Diana Spencer for several years, took a serious interest in her as a potential bride during the summer of 1980, when they were guests at a country weekend, where she watched him play polo.
Split into simpler sentences: The Prince of Wales, had known Lady Diana Spencer for several years. He took serious interest in her as a potential bride during the summer of 1980 when they were guests at a country weekend, where she watched him play polo.