Antiquities, to What Degree are False Provenances Endemic?

David Knell (Thursday, 4 September 2014. 'British history revamped by London road works during the 1970s') makes an interesting point about false provenances on the basis of a pair of 5th - 6th centuries AD northern Syrian lamps sold by a UK dealer as 'British found':

The stories weaved to launder 'high-end' antiquities are old news but these lamps demonstrate just how far some dealers are prepared to go in fabricating the provenance of even minor items. Not content with a mere ""British found", it seems this seller has happily invented a place (London), a time (1970s) and an event (road works) to increase plausibility. Just how much faith can we place on mere hearsay, whether it is a dealer's undocumented claim of provenance when selling an item or a person's undocumented claim of a findspot and circumstances when getting an item recorded in the PAS database? There is much to be said in favour of Elizabeth Marlowe's contention (Shaky Ground: Context, Connoisseurship and the History of Roman Art. Bloomsbury Academic, 2013) that only 'grounded' (archaeologically documented) antiquities form a truly reliable basis for scholarship; those which are 'ungrounded' (lacking archaeological confirmation) can be risky and, if the stories attached to them are simply taken at face-value, may be thoroughly misleading.