During a rehearsal, conductor Sir Thomas Beecham, who died 40 years ago, thought that his female soloist was playing less than adequately on her fine Italian cello. He stopped the orchestra and declared: "Madam, you have between your legs an instrument capable of giving pleasure to thousands, and all you can do is scratch it!" Once he described the sound of the harpsichord as "two skeletons copulating on a tin roof"; on another occasion he declared that "the British may not like music, but they absolutely love the noise it makes". His pointed goatee beard, his proud and portly stature and, most of all, that dry, acerbic wit have passed into musical mythology. No other conductor could possibly have got away with saying: "There are two golden rules for an orchestra: start together and finish together. The public doesn't give a damn what goes on in between."

Beecham's talent for aphorism risks overshadowing his achievements as a a musician. But musicians who worked under him - and orchestral players are often a vituperative lot - still recall "Tommy" with extraordinary fondness. His widow, Shirley, Lady Beecham, is keen to put the record straight. "The jokes were mostly light relief at the beginning or the end of a rehearsal," she points out. "They were just asides. You only have to play a record to know that his musical gifts were something special."

Lady Beecham met her husband when she went to work for the Royal Philharmonic Orchestra's administration nearly 51 years ago. "He could talk about any subject in any sphere," she says. "If you went out to lunch with him, you'd have a most entertaining time as he'd expound on current events. He wrote letters to the papers at great length - and he used to ring up the editors to say that he would like his letter printed tomorrow, and they'd say, 'Yes, Sir Thomas'."

Beecham's route to fame was typically eccentric. His grandfather was the inventor of Beecham's Pills; his son Joseph, Thomas's father, joined the firm and became a successful businessman in St Helen's. On Joseph's death in 1916, Thomas faced an estate complicated by a contract that his father had signed to purchase the Covent Garden Estate, including five theatres, from the Duke of Bedford - due to the war, it took about six years to unravel the legal quagmire. The story goes that a puzzled banker stopped Tommy in the street and asked, "Do you owe, or are you owed, £2m?" Beecham replied: "The answer is affirmative - in both cases." Perhaps it was the Beecham family's ability to think big that gave Thomas his impetus to become an artistic mover and shaker on a grand scale.

In 1899, Hans Richter, due to conduct the Hallé Orchestra in a concert in St Helen's, fell ill; Joseph Beecham, who was mayor, declared that his prodigiously talented 20-year-old son should step in. From there, the young conductor - entirely self-taught - moved on in leaps and bounds. He founded a Beecham Symphony Orchestra in 1909 and a Beecham Opera Company in 1915. At the Royal Opera House, Covent Garden, he conducted the UK premieres of Wagner's Die Meistersinger and Richard Strauss's Der Rosenkavalier, Elektra and Salome; during the 1930s he presented the greatest singers of the day there, including Lauritz Melchior, Lotte Lehmann and Kirsten Flagstad, raising sponsorship himself. He founded the London Philharmonic Orchestra in 1932, and in 1946 the Royal Philharmonic Orchestra. He excelled in Mozart, French music and a range of German repertoire, but had a blind spot over Bach: "Too much counterpoint," he said, "and, what is worse, Protestant counterpoint."

"Between the wars, he did so much for British music," says Lady Beecham. "He championed many wonderful composers, especially Delius. And he helped many institutions by not claiming a fee from those who could only just afford to pay their players, and by persuading wealthy friends to donate money."

Beecham also faced controversy. His finances and personal life were often respectively precarious and volatile; and he is sometimes criticised for decamping to the US during the war - though Lady Beecham defends him by pointing out that he had long-standing engagements to honour in Australia and the US in 1940 and then couldn't get home across the Atlantic. Before that, in 1936, he had taken the newly founded LPO to Germany, where Hitler was in the audience. Why did they go? "He was proud of them," says Lady Beecham, "and he wanted to take them to a country where there were many fine orchestras, to show them what a fine orchestra it was." The story rings of an extraordinary figurative nose-thumbing that only Beecham could have carried off. Dr Berta Geissmar, personal assistant to conductor Wilhelm Furtwängler, was Jewish and had fled the Nazi regime for London, where Beecham promptly employed her as his secretary. "And he took her with him to Germany," says Lady Beecham. "She was absolutely terrified for the whole tour that they'd take her away. But with him beside her, they could do nothing at all." At the Berlin concert, when he saw Hitler applauding, Beecham turned to the orchestra and said, "The old bugger seems to like it!" The remark went out on the radio across Europe. Had Beecham really forgotten that the concert was being broadcast?

To commemorate the 40th anniversary of his death, Warner Music Vision has released videos of Beecham conducting the Chicago Symphony Orchestra. These performances display Beecham's ultimate secret weapons - his innate musicality and his exceptional ability to communicate with his orchestra. "If you look at his eyes in the videos," says Lady Beecham, "you can see that they were telling the orchestra everything. He used little movements, but controlled everything completely with his eyes. He just enjoyed making music."

Richard Walton, former trumpeter, London Philharmonic Orchestra and Royal Philharmonic Orchestra:

"Beecham was the most musical person I've ever met - everything was marked by his gift of wonderful and charming musical feeling.

He could be quite a tartar, but he had wonderful humour and such knowledge - he'd read all the classics by the age of 15, in various languages. Some of the German conductors just beat the orchestra into submission until you did what you were told - he wasn't like that. But he could be terrifying. Once the dancers in a performance of Aida said the music was too fast. That just made him conduct even faster the next night!

When we went to Germany in 1936, the whole place was full of swastikas and for a fortnight there were dinners after the concerts. There, he had the ability to reply to the Nazis with such authority that they listened to him very intently. It wasn't possible to put Beecham down."

Denys Darlow, former orchestral organist, BBC Symphony Orchestra:

"I had the privilege of playing half a dozen notes fortissimo in various works - the rest of the time I just had to be there and watch. It was like having a lesson; one learned so much about the flexibility of speeds from Beecham and the amount of poise he injected into Mozart.

He was a man of very few words, but totally aware of everything that was going on. Once we recorded a concert at the Maida Vale Studios and I was playing only in the second half. In the first half I sat at the back - there was no audience, just the orchestra and a red light. A moment before the piece started, Beecham stared straight at me. Afterwards, one player said: 'He played that piece to you, because there was no audience and he doesn't like red lights.'He wanted to make music, not teach people what to do."

Gwydion Brooke, former first bassoon, LPO and RPO:

"My first contact with Beecham was when he was conducting my father Joseph Holbrooke's early operas. Later a relative advised me to audition for Beecham. He was a cheeky chappie! He put some music in front of me and I said, 'I'll have a stab at it.' He roared with laughter and said: 'Just like his father!'

We all got on well with him - he was a great humorist. He'd turn round and talk to the audience and have them in fits of laughter. Other conductors bored me stiff. There are some conductors who work themselves into a position they shouldn't have at all. Sir Thomas was of a different calibre."

Beecham and the Chicago Symphony Orchestra Parts I and II are out on Warner Music Vision, each price £14.99.