If a French national museum wishes to buy a work of art at auction, it simply exercises its 'right of pre-emption'. Substituting itself for the final bidder, which is what this means, is less fair than it sounds - word invariably gets out about the museum's intentions and few bother to bid. In France, as in Italy, Germany and Spain, any work of art deemed of national importance cannot legally leave its shores, a circumstance which once again significantly reduces its market value. Good news for the cultural patrimony, perhaps, but rather less cheery for the owner. Britain's approach, in contrast, is the most liberal in Europe - its system of controlling the export of national treasures while acknowledging the property rights of the individual is peerless.