The theme in question was that of Paganini's 24th Caprice – the last of his 24 Caprices.
Rachmaninoff's Rhapsody is a set of 24 variations for piano and orchestra, in a single movement. The most popular variation, by far, is the 18th, which has been used in many films – probably most famously in Groundhog Day.
Andrew Lloyd Webber is one of many other composers (Wikipedia lists over 50) who have written variations on Paganini's 24th Caprice. Lloyd Webber wrote a whole album's worth of variations for cello and rock band, for his brother Julian – according to Wikipedia, as the result of losing a bet over a Leyton Orient football match. At its premiere in 1977, the rock band was John Hiseman's Colosseum II. The opening variation was famously used as the theme to ITV's The South Bank Show.
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