This was one of the older SAQs, Question 9 from the first paper of 2000. Co-oximetry was also asked about in Question 17.2 from the first paper of 2010. Generally, the trainees have been expected to know how the pulse oximeter and co-oximeters differ in their function, and what may give rise to false pulse oximeter readings.
Tremper, Kevin K. "Pulse oximetry." CHEST Journal 95.4 (1989): 713-715.
Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.
Ralston, A. C., R. K. Webb, and W. B. Runciman. "Potential errors in pulse oximetry III: Effects of interference, dyes, dyshaemoglobins and other pigments*." Anaesthesia 46.4 (1991): 291-295.