Nothing new here. I left the RAF in 94, and took on a part-time paid job as an RAF Sports Secretary. My life was subsequently a confetti of Coding Notices, endlessly contradicting each other, moving me up and down the scales ... and as I had to calculate my own pay/tax in my part-time job, it was mind-blowing.
I escaped (I thought) HMRC when they finally told me (around 2001) that my affairs were so simple I no longer needed to submit an S-A return.
I moved to Jersey in 2005. Suddenly, a couple of years ago, despite having an NT (No Tax) coding, HMRC suddenly started demanding S-A tax returns again ... which I did, entering 0 0 0 0 0 and some sarcastic comments in the appropriate fields.
It took TWO years to eventually get the idiotic non-joined-up system to remove me from the system again - in May 2014, some 9 years after I left the UK for good. WTF?