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My recently sold C7 also had zero issues.
There seems to be a mix of people on this thread with no issues while others with burn-in. It may just be that the B6 was a particularly bad model. An LG support person I spoke to did say the screens vary significantly in terms of susceptibility to screen burn with the same content. While that doesn't help, it may mean some people are just lucky/unlucky depending on what end of the spectrum you are within the LG acceptable display manufacturing limits. As for the posts on the C9, I can only hope these are more reliable but you may want to report back in a year to update everyone.
Clearly some people will watch channels with logos for hours, have set brightness/contrast etc to higher levels or used settings that do this by default but without any warnings in the settings should a user have to really monitor their viewing? I think it's reasonable to expect your TV to be able to support watching normal TV (which could be 24 hour news if that's your thing!). TVs are not expert user equipment so the user shouldn't need to understand the danger of watching channels with logos/banners etc especially as these are very common.
If you can believe the posts in AVForums regarding a court case against Currys (DSG Retail Limited) it seems a judge agreed and ordered a partial refund.
Given John Lewis' position I assume this no one has done the same to John Lewis. So if you don't believe that your TV was fit for purpose that's one route and/or you're welcome to report as in the Sundays Times OLED thread, regardless of where you bought it from.