This is probably blindingly obvious to some, but wasn't to me ...
We have an ordinary flat screen TV which receives little use, but looks OK when it is used, until I started watching world cup games, and realised a strip of image is being cut off, both sides. Ah! Aspect ratio. No - even at 4:3 a black strip at both sides but the picture itself incomplete. RTFM no good, that just talks about plugging stuff in and basic tuning. OK, much digging through on screen menus and adjusting, to no avail.
Google to the rescue. The setting I needed was within the Thompson set top box, not the TV itself. Reset that, and all is well. Must have had that box for 15 years, and had no idea it did anything other than channels. Hey ho.