I believe the episode is called "How Does Your Garden Grow?" Although no particular fragrance is mentioned, Hastings and Japp mistake Poirot's scent for a woman's perfume so that steers us away from the stronger, typical masculine choices. We also must consider what was available at that time (1930s). Some quick research points towards two suspects: Eau de Quinine (described in reviews as floral, powdery and sweet) introduced in 1898, or Eau de Portugal (an orange blossom take-off on the classic eau de cologne), perhaps catching Poirot's eye upon its introduction in 1938. Of course, it may have been a now discontinued fragrance. But given these two, and knowing how much Poirot wanted to be seen as a proper Englishman, I would vote for Eau de Quinine.
In any case, I would encourage everyone to try the generous sampler pack Trumper sends out essentially for the price of postage. It includes 7 cologne samples, 6 shaving cream samples and 2 skin lotion samples. If you visit the website, scroll down under "Product Type" and click on "Brochure".