Nice to know I wasn't wasting anyone's time then ;-) It was hard to gauge how much blood there was - perhaps 100ml in total (including in the A&E waiting room) and it took 10-15 minutes. As I mentioned, I had had previous instances of blood in my sputum, but this was the first frank blood. Not frothy (what would that have indicated?), but the ambulance crew did note that there were clots. One factor in my triage once we arrived was that I had been on a plane earlier that day - they were concerned about a possible PE.