Imagine ... pounds 120,000 for these

It must have been a lovely sweater for John Lennon to want it so badly: hand-knit, in a pure canary yellow, with twiddly woollen motifs

Imagine the scene: two Liverpudlian art students listening to Buddy Holly records (this was 1959) and haggling over a yellow sweater. "I really want it," says John. "It would be so damn cool to wear to tomorrow night's gig." "Weeeell ... " says Helen. "Go on," says John. "I'll swap you my sketchbook."

That clinched it. Helen Anderson, 16, peeled off her yellow sweater, and John Lennontook out his secondary school sketchbook, and they swapped.

Not a bad investment, as yellow sweaters go. The jumper has probably long ago unravelled, but the sketchbook is still very much here - and valued at up to pounds 120,000 by Sotheby's, which is scheduled to auction it on 14 September. Ms Anderson, now a Cheshire fashion designer, has finally decided to sell because it has become so valuable.

Yet she has cherished it ever since the days at Liverpool Art College when she took in Lennon's trousers to make them into drainpipes and watched him play impromptu concerts with a boy called Paul McCartney.

For fans of the Beatle shot dead in New York in 1980 there could hardly be a more precious item. The sketchbook contains 21 cartoons of Lennon's teachers and fellow pupils at Quarry Bank High School, and his own self- portrait aged 15.

Many teenagers would have had visions of themselves as the Elvis-type, but young Lennon was brutally honest. "Simply a simple pimple shortsighted John Wimple Lennon", he captioned the self-portrait, squinting behind horn-rims with a quiff. The cartoons brilliantly catch the spirit of his teachers and friends. The school groundsman, Albert Yoxall, is shown digging vacantly under the caption: "A simple soul on me farm". His best friend, Pete Shotton, is shown shaking a rattle, and a beefy fellow pupil is lampooned as "Pysche".

Ms Anderson lost touch with Lennon when she married, but there is a sequel. She had remained friends with his first wife, Cynthia, and was visiting her in the mid-1970s when Lennon rang. He told her she should sell the drawings and make some money - and asked her to send him the art school scarf and a black pudding. She adds ruefully: "To this day I feel guilty that I didn't."