Complex sentence: It was listed on the London Stock Exchange and was a constituent of the FTSE 100 Index but in 1989 it was taken over by a consortium formed by GEC and Siemens which broke up the Plessey group.
Split into simpler sentences: It was listed on the London Stock Exchange and was a constituent of the FTSE 100 Index. In 1989, it was taken over by a consortium formed by GEC and Siemens which split the assets of the Plessey group.