It's Ravel Day all next Friday on Radio 3, and by way of a preview, I've been to see one of the British Library's Ravelian artefacts for this week's Music Matters.

I was able to look at his letters - taken from the British Library's archives - written to the composer he was proud to call his pupil - even if the student was three years older than the teacher: Ralph Vaughan Williams. What's wonderful about the three months that Ravel and VW spent together in 1908 is how much they tell us about the cultural politics of the differences between French and English music at the time, and thinking about their relationship gives you a new way of hearing their work - especially Vaughan Williams's.

VW knew that he needed something else in his music apart from the Germanic stodge he had been taught by Stanford and Parry, and thanks to the critic Calvocoressi, he found in Ravel exactly what he was looking for. After a rocky start, at least: when they first met, Ravel asked him to compose a minuet in the style of Mozart, to which VW gave an Anglo-Saxon response. But he was then allowed to concentrate on more creative lessons in France. That meant clarity of texture, limpidity of harmonic language, and luminosity of orchestral and instrumental sound. For VW it was an epiphany that showed him he didn't have to become a British-Teutonic epigone, but something more original.

You can hear the influence of Ravel's teaching and his music in the clarity and economy of the Pastoral Symphony and the Fifth Symphony, and in general, in the new confidence in his own style that VW found in his music. So much so, that Ernest Newman heard VW not as a "British composer" but as a musician who had been dangerously influenced by those dastardly Frenchies. That's a wonderful piece of reception history, because it shows how inaccurate our preconceptions of composers really are. We think of VW as the quintessential embodiment of musical Britishness, but that's not how he was always heard at the time, and there's a far greater continental influence in his music than we're prone to hear.

Ravel's friendship with VW lasted, in the letters, until 1919 (only Ravel's side of the correspondence survives), and their closeness is revealed in missives like this postcard from June 1916. (There's also an earlier postcard in the collection that Ravel sent from Newcastle, asking to be put up by the Vaughan Williamses at their home in Chelsea: Ravel on the Tyne sounds unlikely, but it happened!) Ravel had wanted to be an air-bomber, but was rejected because he was too small; he was finally allowed to become an ambulance driver, and he saw and experienced the horrors of the front-line at first hand (the postcard tells Vaughan Williams that his vehicle had broken down, and that he is 'very tired' - a euphemism for the terror we now know he went through). VW was a stretcher-bearer, who also knew the unimaginable tragedies of the trenches. And both of them made their war-time experiences part of their music in ways that are all the more moving by being implicitly felt rather than explicitly exposed: Vaughan Williams in his Pastoral Symphony, and Ravel in his Tombeau de Couperin.

There were non-musical sides to their friendship, however: Ravel had a penchant for steak and kidney puddings on Waterloo station, and in Paris, Ravel took VW off to see some "jolly tarts", in his phrase. VW returned from the experience with his honour intact, seeing the women as extras from a Toulouse-Lautrec painting rather than as potential conquests. If the musical influence seems like one-way traffic from Ravel to VW, Ravel at least admired the Englishman's music enough to give the French premiere of On Wenlock Edge (a performance that VW later remembered as one of the worst he had ever heard), and did what he could to promote his music in the chauvinistic context of French musical life. VW's music would have been very different without Ravel's example, and without Vaughan Williams, Ravel would have lost one of those rare things in his life, a close friend.