Now that I've checked a bit further I'm fairly sure this must be the BAS's VP-FAL, which was withdrawn from use at Deception Island during the 1966-67 season. It seems that the first two BAS Otters, 'FAK and FAL, did not carry their civil registrations, but had RAF roundels and factory serial nos., while the last one, 'FAM was civilian-marked. 'FAK and 'FAM both crashed, so my money's on this being 'FAL. Mystery solved?