In 1973 the popular Secret Lemonade Drinker advertising campaign was launched, devised by Rod Allen who wrote the slogan. The adverts featured the actor and mime Julian Chagrin, dubbed with the voice of Ross MacManus (father of Elvis Costello, then called Declan MacManus). MacManus wrote the song, with his son providing the backing vocals.

“1973 saw the launch of the brand’s most famous advertising campaign, ‘The Secret Lemonade Drinker’, which remained on screen until 1984. The ad featured a man in striped pyjamas creeping downstairs to raid the fridge for R Whites Lemonade. Ross MacManus (the father of singer Elvis Costello) wrote and sang the original song with his teenage son, providing backing vocals. The ad won a silver award at the 1974 International Advertising Festival.” 

The lemonade drinker is often erroneously identified as John Otway, though Otway did appear in a later R. Whites advertisement in which he drank it in a phone box.

Info gleaned from Wikipedia