It seems remarkable that instrumental film themes that have made Number One in our singles chart can be counted on one hand. The last of these was 50 years ago, when Ennio Morricone's theme to The Good, the Bad and the Ugly topped the chart for four weeks in November. It's a re-recorded version by a different orchestra with a steady beat underneath, rather than the dramatic opening to the film, but it's Morricone's tune.

In fact the only other was the theme music to the 1949 film The Third Man, also known as The Harry Lime Theme. Many would argue today that it's one of the best films ever made and yet the theme music is often the first thing people will mention. Set in the ruins of occupied Vienna under the shadow of black market racketeers it's a jaunty tune played on an Austrian zither - and nothing like the bombastic, melodramatic orchestral themes that opened most films then.

You could stretch a point for Floyd Cramer's On the Rebound from 1961, but only if you allow that the film in which it features - An Education - wasn't made until 2009.

Moon River from Breakfast at Tiffany's was also Number One in 1961, but this was a version of Audrey Hepburn's song with vocals rather than Henry Mancini's instrumental theme.

The same is true of television themes: hugely popular and memorable, yet the only instrumental theme to top the charts was Eye Level in 1973 from ITV's Amsterdam detective series Van der Valk.

While instrumentals of any kind rarely become hit singles, soundtrack albums have always sold very well despite most film scores being - by definition - background music. I always supposed they were purchased mainly as souvenirs of well-loved, popular films rather than for the thrill of the music alone. (I exclude recordings of "musicals" such as Night Fever and Grease here since these are effectively collections of conventional pop songs.)

I suppose you couldn't sell a million or more singles on the basis that they were a nice souvenir to a film, not when the Beatles, Cliff Richard or Slade are releasing hit after hit at the same time.

I'm not sure it's worth pondering for too long though. The one thing these four Number One tunes all have in common is simple: they're all brilliant, memorable, hummable tunes. Pop music, in other words.