Fiona Bruce and Philip Mould investigate the provenance of a painting with a Venetian view, trying to ascertain whether it is the work of Francesco Guardi or Michele Marieschi. Owner Nick Hopkinson inherited the painting from his great grandfather and Guardi and Marieschi are both known for their depictions of Venetian views and were contemporaries in the 18th century of Canaletto. But there's a big difference in value between them - if Nick's painting is a Marieschi it could be worth half a million pounds, if it's a Guardi it could be worth up to £10million.