Hard by the Rialto, in a densely packed and depressingly tacky quarter of Venice, the church of San Giovanni Cristosomo houses one of Giovanni Bellini’s most luminous and exquisite paintings. ‘I Santi Cristoforo, Girolamo e Ludovico di Tolosa’ is known to locals as ‘the Burger King Bellini’, after the fast food outlet opposite the church door.