LONDON (Reuters) - The three surviving members of the Beatles will appear in public together for the first time in almost 30 years on Monday at a memorial service for Linda McCartney. George Harrison and Ringo Starr will join Sir Paul McCartney and about 700 other guests for a London church service for Linda McCartney, whose sudden death from breast cancer in April ended one of the closest marriages in show business. The congregation at St Martin in the Fields church in Trafalgar Square will sing „Let It Be“ – the moving ballad McCartney wrote for his own mother Mary who herself died of breast cancer when he was 14 years old. The last time the Beatles sang in public together was in 1969 when an impromptu session on the rooftop of their Apple record label building in central London brought traffic to a standstill. Linda McCartney died in the United States at the age of 56 after a sudden deterioration in the cancer she had been battling for two years. McCartney has shut himself away since returning to the farmhouse home they shared in Sussex, southern England. McCartney has not appeared in public since the death of his wife, who he described in a poignant statement in April as the love of his life. Linda‘s body was secretly cremated immediately after her death and her ashes scattered over the family farm. Linda McCartney was a photographer and a vegetarian who started her own successful food business and was a strong supporter of animal rights. Several thousand animal welfare campaigners are expected to hold their own candle-lit vigil outside the church on Monday. Friends said McCartney would deliver a tribute to Linda during the service, which he wanted to be a mixture of mourning and celebration of her life. Television crews and photographers were banned from the interior of the church. The McCartney‘s three children – Mary, fashion designer Stella, and James — were to be present along with Linda‘s daughter Heather from her first marriage. Friends said John Lennon‘s first son Julian might attend, although it was not known whether his widow Yoko Ono — with whom McCartney had a lengthy feud after the 1970 break-up of the Beatles – would join them. Paul and Linda, who played and sang together in the 1970s band Wings after the Beatles dissolved, recorded six songs written by her shortly before her death for a planned tribute album.