Update: In a sign of the strength of feeling in the Lords on this matter, Boothroyd's motion rebuking Cameron passed by 177 votes to 29. This foolish, unforced error now promises to make Cameron's life more difficult than it needs be between now and March next year.
What should worry Cameron most is how cross the peers, including the Tories, are about the whole situation. A considerable number of Tory peers, including some you would view as Cameron loyalists, are intending to turn up and vote with Boothroyd tonight. This makes a government defeat more likely than not.
Ministers, though, seem surprisingly unbothered about the prospect of losing this vote. I suspect that this is because the motion isn't binding. But it is never a good idea for any government to alienate the House of Lords. They have a way of making the executive pay for this kind of arrogance.