This is a quote from the article you linked:
"In 2007 when she was recruited to look after his London flat he was given a copy of her passport and a Home Office letter stating that she had indefinite leave to remain in the UK and the right to work and run a business.
Last month – mindful that he was steering the immigration bill through the Commons and publicly warning employers of the need to check the status of employees – he sought fresh evidence.
On Thursday morning, he said, he was informed after checking with immigration officials that she was in the country illegally."