The National Symphony Orchestra took London by storm Saturday night, playing the world premiere of the latest work by England's foremost living composer, Sir William Walton.

Mstislav Rostropovich opened the concert in the Royal Festival Hall with "Prologo e Fantasia" which the orchestra commissioned from Sir William last year. The completed score was delivered only three weeks ago in Zurich, the first stop on the NSO's current European tour. The conductor and orchestra learned the work in two rehearsals in the crowded weeks since then. Saturday night they played it publicly for the first time. It provided a brilliant opening for the orchestra's Royal Festival Hall concert. The occasion was heightened by the presence of the composer, who turns octogenarian next month.

Walton applauded enthusiastically from his box and stood several times to acknowledge the audience's ovation.

The new work is brief, lasting only around six minutes. It opens with a bold upward figure from all of the strings which slowly subside into a brief meditative passage. The fantasy then follows, reminiscent in a way of the great night interlude from Walton's opera, "Troilus and Cressida."

The fugue is brilliant in brevity, a series of mounting eruptions of stunning power. In a way, the work is reminiscent of the manner of the orchestral essays of Samuel Barber. It is a work that should very quickly enter the standard orchestral repertoire.

Sir William was delighted with the performance, saying afterwards in a brief conversation that he thought the orchestra played it wonderfully.

The entire concert was one of the orchestra's most brilliant on this tour. The Royal Festival Hall's acoustics are very clear and dry. The sound from the stage is enormous but, somehow, does not seem to come out and surround the audience. A comment from the late Sir Thomas Beecham illustrates the general feeling of musicians who play there. Sir Thomas told me shortly after the hall opened, "But I don't want to hear everything that is going on in the orchestra while it is playing!"

While the NSO musicians agreed on the acoustical properties of the hall, they agreed also that they liked playing there. The result was the finest Shostakovich Fifth of the tour, following the notable success of the Walton, and a polished Eighth Symphony of Beethoven.

The National Symphony Orchestra has certainly made its mark in the major musical capitals of Europe. It now heads for Brussels, Hamburg, Barcelona and Madrid.