The final derivatives of the XJ12 and Double Six models arrived in 1979, the Series III. Like the six cylinder Series III cars the Double six and XJ12 benefited from the clean-cut, new body designed by famed Italian design house, Pininfarina. The new body featured more glass, slightly squarer lines around the roof area, plus new front and rear end styling.
The XJ12 and double six were still hampered by poor fuel consumption (13mpg at best) until the HE models arrived in 1981. The HE model (or High Efficiency) produced more power (299bhp) and could return slightly better fuel consumption figures of around 16mpg. The XJ12 and Double Six Series III soldiered on until the late 1980's, outliving the six cylinder XJ6 and Sovereign due to problems with installing the engine in the new XJ40 series.