@IamJai: Sorry to hear that your nan is experiencing this issue with her TV. Are you able to confirm the full model number, located on the sticker on the back of the device? In the meantime, try powering off the TV and unplug it at the wall for around 30 seconds, then plug it back in and power it on using the remote. Does the issue persist? If so, head to Settings > Support > Self Diagnosis > Picture Test. Are the lines/flickering present in the sample image, and do you get any 'Failed' messages in the summary at the end?
If you're looking at purchasing another TV, but you're not sure what to get, then please check out the link below and use the filters on the left hand side, and I'm sure you'll find what you're looking for.
Hi @IamJai If still looking should be able to get your Nan an upgrade that is not too complicated, even if Tvs are more advanced these days. There are few 40 inch UHD Tv's these days, Samsung's models from the last few years start at 43 inches. If you have purchased hopefully all is well. Otherwise think the TU Models might be suitable, a few reviews in this article for anyone looking at entry models. https://www.whathifi.com/best-buys/tvs/best-samsung-tvs