Firstly, I'm an idiot. I'll take that as read!
In October 2017 I did some admin work on a specific project, on a self employed basis. Was a tiny job and I earned about £1200.
That was my only income in that tax year.
At about the same time last year the same organisation took me on for several larger projects.
I've just been looking over the rules for self assessment as I would expect to need to file a return this time next year and it seems that because I earned more than £1000 in 2017-2018 I should be filing a return now for that income. I'm astounded that that threshold is so low given my tax allowance is £11850.
It's no problem to file a return as it's very simple, but it seems I should have registered to do this last October!
Am I misunderstanding this? It seems odd to me to file a return (presumably making work for the HMRC) which won't produce any tax payment.