Fiona Bruce and Philip Mould investigate a delicate sketch depicting a mother and child, purported to be by one of the modern art world's most famous names, Amedeo Modigliani. Its owner, Henrietta Sitwell, inherited the work and always believed it to be genuine. However, a leading auction house recently cast doubt on its authenticity. If the work is genuine, it could be worth up to £100,000. If not, just a few hundred.