Picture size adjustment on old Beko Model. 12220?

The picture on my 16yr old 20inch tv has crept out of screen view at the edges. That is - top, bottom & sides. Its frustrating when text is shown with the picture because the text cannot be read, likewise the channel logo's or numbers in the corner cannot be read either.
I know a simple adjustment will return it to normal - albeit temporary as the the set may be on its way out. I've done this on other tv's years ago but I just cant remember what or where to adjust it from. There are no accessable controls on the case of the set but there are several potentiometers on the board attached to the tube and a few more on the main board. There is also 2 other adjusters which stick out of something with HT leads coming out of it to the screen. Any advice be be appreciated