The second Britten is on stage at the gala reopening of Snape Maltings Concert Hall in June 1970 following a devastating fire the previous year. His hair is grey, his face puffy, his demeanour that of an old man. He is dressed as though for an investiture (appropriately, since the Queen is in the audience), and although his performance of the two central movements of Mendelssohn's Symphony No. 3 is a model of fire and restraint, he will finish the concert conducting scenes from his opera Gloriana, head in the score, sweat pouring off his forehead and fogging his heavy glasses. Sadly, in 1970 he was very ill but did not yet know it. Two years earlier he had been diagnosed with endocarditis and had been treated aggressively with penicillin. He had seemingly recovered, but his heart was weakened by the infection and a diagnosis of aortic incompetence in August 1972 would leave him requiring a heart transplant. Those close to him did not notice his physical decline; it is only the juxtaposition of these two performances that makes it so evident.

Although his illness inevitably affects the way we now listen to his performances from the late 1960s and 1970s and think about the pieces he was writing, Britten's music-making was actually undiminished until 1973 when the stroke he suffered on the operating table put paid to most things other than laboured composition. He was anyhow before then a slightly awkward conductor. Marjorie Fass, part of the Frank Bridge circle and a mother figure to Britten in the 1930s, described him in 1938: 'If he goes on for ever he'll never be a conductor – you never saw anything so stiff & held in.' He developed greater physical fluency over time, but the point of Britten's conducting was never how he looked when doing it; instead it was about the sheer musicality he brought to the task, which British orchestral players, raised on an undernourished diet of blustering conductor-knights, greatly appreciated. Nor did he indulge in the podium antics of Leonard Bernstein – the big leaps, the cocked eyebrow and sly smile; Britten guardedly admired his American contemporary, but there were too few overlaps in temperament for them to be friends, let alone rivals. Britten was polite and direct with his players, instructing them in an accent that had long lost any hint of his Lowestoft childhood, phrasing them with exquisite grace. Arguably his physicality occasionally hampered him in a way it never did at the piano. But these were rare occasions. And he compensated for any physical awkwardness by working time and again with the same players who read him fluently. The English Chamber Orchestra was a particular favourite.

Britten's performance of Mozart's Symphony No. 40 in 1964 encapsulates so many aspects of him as conductor. It is revelatory footage, with close-up frames of his unsmiling face, and a more modern conception of filming an orchestral concert than anything previously featuring Britten. There are lovely details – including the moment when he wipes his brow with the back of his baton hand, midbeat – and what comes across in this unfussy performance is Britten's admiration for a piece he had known well since childhood. ('Mozart's superb G. min', he wrote of a performance by Bruno Walter in 1934, 'surely the loveliest bit of music ever conceived.') It is a performance of its time – not simply because of the footage of young, jacket-wearing men in the audience sporting Beatles' haircuts – but because of its gentleness, subtleness, and generous string vibrato; a new breed of Mozart conductors was just round the corner and although they would replicate the small string section Britten here uses (eight first violins, six second violins, four violas, four cellos and two double basses), their driven speeds and often arid sound would set them apart.

Britten is similarly undemonstrative conducting his own Nocturne in the same concert – an evocation of night thoughts and moods of all kinds. There is the occasional flash of eyes or jab of finger, but mostly in this difficult score he impassively shepherds his players, who reciprocate by counting like mad. He stands slightly aloof even when conducting his great setting of Shakespeare's Sonnet 43 ('When most I wink, then do mine eyes best see'), leaving the music to do its magic. Pears sounds fresh and dynamic and although he clutches a vocal score – a prop against his unreliable memory in this decade – he does not use it. This performance complements Britten and Pears's spacious and colourful studio recording of the Nocturne by giving us an intimate look into how the musicians worked together in public – the unfailing ensemble and support, Britten's look of admiration as he shakes Pears's hand at the concert's end.

The footage at Snape is equally revelatory. The honeyed colour is no freak of film or laboratory; this is how the Maltings looks under lights to this day. And it is not simply colour film that makes the performance seem so much more contemporary than the 1964 concert: even in such a short time, style has changed and the orchestra, not in concert dress, embodies a marked sense of individuality, the great prize of the 1960s. There are Britten trademarks – subtle phrasing, poised rubato, palpable structure and drive – and it is these qualities that should be remembered, not the appearance of the twentieth century's consummate musician in decline.

Paul Kildea