Covent Garden is a difficult place to grab something to eat. The huge numbers of people crammed in even on weekdays means that, for most places, booking is a necessity. If you are willing to go around the corner from the congested Tube station, then you might find an alternative in Le Palais du Jardin. You will still have to book in the evenings, but it is easier to get a late reservation than right in the heart of Covent Garden.