A man with dementia has won plaudits on social media after a viral video showed him playing a piece he composed decades ago.

Paul Harvey, 79, wrote the music for the song "Where's The Sunshine" in the 1980s, when he was head of music at Imberhorne School in East Grinstead, West Sussex.

To his own surprise, he had no problem recalling it note for note when visiting his son Nick in Crowborough, East Sussex.

“Dad has dementia. Sometimes he drifts into another world and I feel like I'm losing him," Nick, who writes music for television, tweeted, along with a video of his father playing the tune.

“He is never more present, however, than when he plays the piano."

The video received more than half a million views on Twitter in less than 24 hours, while the piano score for the tune has been requested many times.

The post also grabbed the attention of singer-songwriter Emeli Sande, who described the piece as “beautiful," while deputy leader of the Labour Party Tom Watson also shared it.

“Thank you @mrnickharvey for sharing this personal moment," wrote Sande. “Your dad's composition is beautiful. Listening to him play, I am reminded of the healing power of music and the refuge it provides for us. Love to your dad and family and all others suffering with dementia."

Paul attended the Guildhall School to study music when he was younger, and in the 1960s and 1970s worked as a concert pianist and composer.

It was when his son Nick was born that he decided to go into teaching to obtain a more reliable income, writing the music for "Where's The Sunshine" – the finale in a school production – with head of drama Pete Talman writing the lyrics.

“I don't know what to say, I have always shunned publicity," Paul told the Press Association. “I've got dementia and you just keep going! As long as I've got access to my piano, that's the main thing.

“I am very pleased that something I've written has had such an effect. It's come so late in life, it shows that it can happen at any time."

Sheet music for "Where's The Sunshine," music composed by Paul Harvey (Nick Harvey/PA)

Music is regularly used as a form of therapy for dementia patients as it is believed to reach parts of the brain that other forms of communication are unable to.

And the positive effect that recalling music from the past has upon Paul seems clear to his son.

“I try to get him to play the piano because he seems to become much more connected, focused and in the room," Nick told the Press Association.

“Whenever he visits, even before he got ill, the first thing he does is gravitate towards the piano. It's his solace, if you like.

“I'm just over the moon that this song and Dad's talent is being broadcast, because he deserves it."