Tilli
thanks you answered the T**T before I could. Minge Coiffure, don't allow your management to bully you into doing something unsafe. The vast percentage of discretions are caused by poor or unrealistic scheduling. If you are not fit to use your discretion don't do it. If you are worried about getting a bollocking just put a tape recorder in your bag or on the telephone.
If you are fatigued or likely to be, send Crewing an email or a fax telling them so. It is a brave guy who would then ask you to use your discretion or have you for "tea and bickies".
I haven't had to use my discretion for some because in the past we had the same problems as you are having. Because we said NO "they" had to change.
Finally copy your email or fax to chirp