Where for the purpose of assisting with an investigation a person attends voluntarily at a police station or at any other place where a constable is present or accompanies a constable to a police station or any such other place without having been arrested—
(a)he shall be entitled to leave at will unless he is placed under arrest;
(b)he shall be informed at once that he is under arrest if a decision is taken by a constable to prevent him from leaving at will.