With an unprecedented Electoral Commission investigation into the refurbishment of his Downing Street flat, Boris Johnson could have avoided a lot of trouble if only he had followed the example set by one of his illustrious — and thrifty — predecessors. As chatelaine of No 10, Margaret Thatcher applied the same rigour to spending on decor as she did to the nation’s finances. On one occasion, the Iron Lady vetoed plans for a new carpet in her study. ‘There was a small threadbare square in front of her favourite armchair,’ one of her former advisers told me this week. ‘Her officials wanted a new carpet but she was having none of it.’