Patrons of the arts are serving London's art-loving public well. Earlier this year, we told you about Artist Rooms and Anthony D'Offay's donation of his personal art collection. Now, some rarely- and never-before-seen works are on display at the National Gallery and Tate Britain. The late Simon Sainsbury (great-grandson of the grocer) bequested 18 works of art with a combined estimated value of £100m to the museums in 2006. Amongst the paintings are works......