Petera it was 2007, apparently,
In a dissertation:
Truss has long been an advocate for eliminating waste in the government. In 2007, she wrote a dissertation for a reform think tank that claims civil servants fly in economy class. “All public sector workers should feel personal responsibility for the money they spend and the money they save,” she wrote.
“They should at least spend the taxpayer’s money with the care they would give themselves. This change in thinking is reflected in everyday changes such as traveling in the economy rather than in business class. “