Will hmrc be on my case if I do a self assessment then register as CIS?

I mainly keep to myself, really don't earn a lot of money. Literally, all of it is cash in hand. A new job has asked me to do a self assessment then register as CIS. My question is: after I go through this, will HMRC be on my case for decent amount of time even if I leave this job? I've heard you can get fines if you don't submit your tax returns in time? Right now, it feels im invisible to them.

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Lastly, just to save face I do agree with tax but I currently earn next to nothing. I'm in England by the way.