Thanks, EQ! I hope so. On a related note, I'm starting my own business so I might pick your brain in the future about becoming a limited company vs. other options!
I have a small side business as well, and it was my registering as a sole trader that triggered a similar letter, I think. In comparison, my husband, who has a company here, has always does PAYE via his company, but this year ALSO had to self-assess as he received payment from a uni in Singapore that was separate from both his Uni job and his company.
I think there are a few things that trigger letters like that, but it doesn't necessarily mean you owe any money.