http://www.bbc.co.uk/rd/blog/2017-12-morecambe-wise-video-film-archive-restoration

"Imagine painting a beautiful picture on one sheet of glass. However, instead of painting that picture with paint, you use Marmite. Now put a second sheet of glass on top of this - you have a lovely image sandwiched between the two sheets of glass. However, if you were to pull those two pieces of glass apart, the image would be destroyed beyond recognition. That is what we have with Morecambe and Wise – a Marmite sandwich. And we only want the Marmite."