What is the current state of the law on questioning persons in custody?

In the year 1912 there was so much doubt in England among the police due to conflicting authorities about the proper course for them to pursue in questioning persons in custody that the Judges, at the request of the Home Secretary, drew up certain rules for their guidance of which the first four are as follows: (1) When a police officer is endeavouring to discover the author of a crime there is no objection to his putting questions in respect thereof to any person or persons whether suspected or not from whom he thinks useful information can be obtained; (2) Whenever a police officer has made up his mind to charge a person with a crime he should first caution such person before asking any questions or any further questions as the case may be; (3) Persons in custody should not be questioned without the usual caution being first administered; (4) If the prisoner wishes to volunteer any statement the usual caution should be, administered. It is desirable that the last two words (“against you”) of any such caution should be omitted and that the caution should end with the words, “be given in evidence:” Rex v. Cook (1918) 34 T.L.R. 515; Phipson, 8th ed., 1942, p. 251.

The same rules are referred to by the learned Chief Justice of the King’s Bench in Rex v. Bohun (No. 2) 1933 CanLII 237 (SK QB), [1933] 3 W.W.R. 609, and are also set forth in a memorandum addressed by him to the commissioner of provincial police in Saskatchewan, a copy whereof follows that report (see p. 613 ibid.).