On reporting my issue with the 3 metre Focal Headphone Cable to my local (PP) Focal dealer, he invited the Focal representative to his store to demonstrate that there wasn’t a problem with the Cable.
On arrival, I asked the dealer (rather than the Focal representative) to attempt fitting the Cable to my Celestee Headphones: he couldn’t insert the plugs at all.
Fortunately, the Focal representative had bought along a replacement Focal Cable: this inserted correctly into my Celestee on the first attempt. Both parties agreed that there was a visible difference in the width of the plugs between the 2 Cables.
The lesson is this: don’t just accept a dealer or manufacturer’s assertion that there is no fault with any product that you buy that is contrary to your own experience.