I have been living in Germany for three years and working for a London-based company (as an employee) remotely from home. (I lived in the UK between 2012 and 2015.)
I sent P85 to HMRC back in September 2017 and they assigned a tax code "NT" to me in August 2018 and sent me a refund of all the taxes paid between April 2018 and August 2018. (I had also filed tax returns for years ending April 2016 and April 2017 and got refund for all the taxes I paid in the UK while living in Germany, as expected.)
Now I have received a letter from HMRC saying that I haven't sent them a tax return for the year ended 5 April 2018. Question: Why are they asking me for that? I thought that having sent them P85, being assigned code "NT" and being refunded everything they should no longer need me to file a tax return. I paid zero and claim zero, period.