Murder on the Mews

It occurs to Hastings that with fireworks going off all around London on November 5th (Bonfire Night), no one would take much notice of a shooting. And within hours Poirot finds himself investigating a violent death in Hastings's own mews garage. At first, it seems as if a young lady who was about to marry a self-important member of parliament killed herself - but Poirot is not happy with the suicide theory. So if it was murder, who is the killer?