Supermal
Sorry, cant really help you.
The fault appears to be on 50Hz input to the set. So in my case I have adjusted the OUTPUT display settings from my connected devices. So on my PC i went - settings, display, then was able to change the display output from 50Hz to 60Hz. I was also able to do the same with my Sumvision media player. My BT Youview box does not allow for frequency adjustment but i have not noticed a issue on this.
I only use the Youview box to watch TV & have never used the built in tuner. The problem you have is that as the tuner is built in, there is consequently no " output " setting available to adjust.
In other words, the built in tuner is fixed at 50Hz. This flicker issue was introduced on a software update 6 months ago & no consequent update has fixed it.