I asked Emma and Jack and they confirmed they took their TN name from a photographic company which did big business in the UK from the 1940's until about 1980. I knew of it because my older brother was a keen photographer and he used Gratispool because their system reduced the "running costs" of photography very substantially and you got a replacement film every time you sent a film in for processing. I knew my brother eventually learned to process his own films and stopped using Gratispool. I didn't think much about it at the time, but I eventually realised his decision was more connected with a girlfriend who like posing nude than with any dissatisfaction with Gratispool.
For those who may be interested this link tells the story of Gratispool:
https://www.photomemorabilia.co.uk/Gratispool.html
Pamela