It sounds like the YouView box is outputting a widescreen image, and because your TV's has an aspect ratio of 4:3, everything looks stretched vertically.
I've had a look at the manual for TV too but there don't appear to be any options for selecting the aspect ratio. I've also looked at the YouView box manual expecting to see the same sort of options I get on my Sky Box such as 16:9, 4:3, 4:3 letterbox, etc. The last two are the ones relevant to your setup. I can't unfortunately find the same settings for your YouView box. Have a good look yourself and hopefully you'll find there is indeed a way to change the box's aspect ratio to 4:3.