I have purchased the above model TV and the picture quality is very poor in normal

TV mode. The picture is very clear on HD channels and also Netflix etc, but very grainy/fuzzy/blurred on terrestrial TV. The signal quality and strength is 98% so the digital signal doesn't seem to be the issue and I have no other equipment connected to it. For a £600 TV I'm disappointed with the picture quality, any ideas how I can improve it please?