I didn't know this, its from http://www.thisdayinmusic.com

The 'Secret' Number One
 - John & Yoko and The Plastic Ono Band - "Happy Christmas (War Is Over)"


There was a time when no new chart was compiled over the Christmas period; it was a labour-intensive process requiring couriers racing around the country, collecting sales diaries which were then delivered to a data-entry team who would manually key the catalogue numbers into an early computer system. These staff needed a week off for Christmas. Furthermore, the media didn't seem to require a chart update due to lack of publications and special festive programming.

On 8th December of that year John Lennon was assassinated.

On the week ending 27 Dec, St Winifred's School Choir hit the top spot with "There's No-one Quite Like Grandma." But, directly over Christmas week, large numbers of shoppers bought Lennon's classic "Happy Christmas (War Is Over)."

The chart compiled (though not published) for w/e 3rd Jan 1981 had John Lennon at number one. By the time normal service had been resumed (w/e 10th Jan 1981), John Lennon's "Imagine" had climbed to the top in any case.

Yes, there was also an instance in 1976 where the British chart compilers got it wrong...

An adaptation of 'Rodrigo's Guitar Concerto' by easy-listening orchestra Manuel & His Music Of The Mountains (Manuel was bandleader Geoff Love) was announced as the #1 on 28th February that year, but some three hours later, the compilers sheepishly admitted that due to a 'computer error', the actual #1 was the same as the previous week ('December '63' by Frankie Valli & the Four Seasons)...

So Manuel was ignominiously dethroned and to add insult to injury, was leapfrogged the following week by 'I Love To Love' by Tina Charles...

It's never been officially recoginised, but those three hours at the top makes Manuel the artist with the shortest reign at #1!