A: Schools vary in the amount of cover they ask newer teachers to do. In some, they're used only as a last resort. In others, their goodwill and enthusiasm are shamelessly exploited. Unfortunately, nobody can refuse to cover. The law is in Teachers' Pay and Conditions. You're expected to do what the head asks and that includes covering for colleagues, for the first three days of their absence. Newly qualified teachers aren't exempt from this, though many schools try not to use them at least in the first term.
Don't let cover eat into your 10 per cent reduced timetable for induction.
If you earmark free periods for this and arrange to be doing professional development, they're less likely to get taken away. Or be bold, and write on the cover slip: "I thought I was on a 90 per cent protected timetable.
However, if there is an emergency, let me know." That's assertive!