In 1999 it merged with BTR plc to form Invensys .
Siebe plc was once one of the United Kingdom 's largest engineering businesses and was a constituent of the FTSE 100 Index .
In 1999 Siebe plc merged with BTR plc to form Invensys .
Siebe plc was once one of the United Kingdom 's largest engineering businesses and was a constituent of the FTSE 100 Index .