Swans usually team up for life, and if one goes missing then there is a worrying reason. If both parents have left, and its obvious that the young are still not able to fend for themselves, then contact the RSPCA or local wildlife trust or charity. I am surprised the bailiff hasn't considered this.
Losing adults can be through various reasons, ranging from animal (same species or otherwise) or human attacks and sickness. To lose two adults should raise alarm bells. Usually the young will move away, not the parents.