Musical anniversaries can be lethal for the music concerned. I was turned off Mozart for years (which was a pity) because I overdosed on his pieces in 1991, the 200th anniversary of his death. I was also sickened by the shameless cashing-in on his fame in Salzburg and Vienna, cities that had treated him disgracefully when he was alive.

No danger of that happening with Ralph Vaughan Williams! The notion that England might overcelebrate its finest composers – let alone flog chocolate effigies of them to tourists, as Salzburg does with Wolfgang Amadeus – would be hilarious if it weren’t tragic. As a nation, we treat our classical music geniuses rather as families treat dodgy uncles. We ignore them and hope they’ll go away.

But Vaughan Williams hasn’t gone away. Amazingly, the anniversary commemorations of his death (50 years ago next Tuesday) have not harmed his music, but enormously boosted its credentials.

Before this year he was mostly regarded as a quirky footnote in musical history – a rumpled, dilettante figure who was inspired by folksong-collecting journeys across southern England to write pastel-shaded musical portraits of a rural England that had disappeared long before his time. Pieces such as The Lark Ascending and the Tallis Fantasia were acknowledged to be charming. But what did they say about the turbulent century in which Vaughan Wiliams lived?

William Mann, the music critic of The Times in the 1960s and 1970s, once wrote that any true masterpiece “must proclaim the date of its composition”. By that criterion, Vaughan Williams was deemed a failure. His music was steeped in the modal contours of folk-song and Tudor polyphony, not the harsh dissonances of the mid-20th century.

But thanks largely to some superlative concerts by the Philharmonia Orchestra and others, the general perception of Vaughan Williams has been transformed beyond recognition this year. Performers, audiences and critics have realised that most of Vaughan Williams’s finest pieces are nothing like the caricature “cowpat” pastorals in which he was thought to have specialised. It has dawned on us that, in such trenchant, restless and violent pieces as the Fourth and Sixth Symphonies and the Second String Quartet, Vaughan Williams was articulating as anguished a response to the traumas of the 1930s and 1940s as Shostakovich or Bartók had done.

But there has also been a recognition of something even more profound in his music. He was an agnostic, rather than a Christian (though he did give the Anglican Church some of its most stirring hymn tunes). But as an ecstatic audience saw at Sadler’s Wells in June, when Richard Hickox conducted two spellbinding performances of Vaughan Williams’s magnificent John Bunyan-inspired opera The Pilgrim’s Progress, there’s a deep spirituality running through his work: a profound belief in life as a struggle between good and evil, or (as in his Pastoral Symphony, an elegy for the Flanders dead, and the Captain Scott-inspired Sinfonia Antartica) between frail man and implacably hostile elements.

That went hand in hand with an old-fashioned liberal humanism.

Vaughan Williams was compassionate, tolerant and utterly decent, and that shines through in his music. Descended from Charles Darwin on one side and Josiah Wedgwood on the other, he tempered his intellectual interests with a huge sense of practical obligation. No great composer wrote more for nonprofessionals.

The climate today is much more sympathetic to these concerns than in the second half of the 20th century. Tonality is back. So is spirituality. So is the feeling that a healthy musical world is not one in which composers churn out impenetrable works for highly trained professionals which music lovers are then expected to pay money to endure. Vaughan Williams’s vision of music as an all-encompassing spectrum – in which young and old, amateur and professional, folk-performer and classical musician all interact – is increasingly seen as the key to a truly musical nation.

There has been one other crucial factor in Vaughan Williams’s rehabilitation – the revelation of his complex and agonisingly frustrating love life. It explains so much about the fierce emotions energising his later music.

Most music lovers vaguely knew that he remarried, near the end of his life, a much younger woman. But John Bridcut’s intensely moving TV documentary The Passions of Vaughan Williams uncovers the full story of what must be regarded, even by the prodigiously amorous standards of composers, as one of the most astonishing love affairs in musical history.

Trapped in a mostly sexless marriage, yet bound by loyalty and the conventions of the period to stick with his morose and increasingly invalid wife, Adeline, Vaughan Williams fell head over heels in love with the vivacious Ursula Wood, a young married woman. He was 65, she 27. Remarkably, they carried on their clandestine relationship for 13 years before Vaughan Williams was free to marry her, which he did when he was 80.

It’s fitting that the BBC is repeating Brid-cut’s 90-minute film on BBC Four next Friday, and a BBC Two airing is also rumoured. Meanwhile, the celebratory Prom on Tuesday includes the voluptuous piece for 16 solo voices that the composer wrote immediately after falling in love with Ursula – the Serenade to Music. Hearing it will be an apt reminder that, with English composers generally and Vaughan Williams especially, apparently still waters run very deep.

The Vaughan Williams Prom is at the Albert Hall, London SW7 (0845-401 5040) on Tues. The Passions of Vaughan Williams is on BBC Four on Aug 29 (8pm)

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