CCTV images showed a gang thought to be of East European origin, possibly a family, walk out with a Louis XVI clock hidden in a bag.
"This clock weighed a considerable amount and we thought would have been extremely awkward to smuggle out. It proves this was not the case, and also shows what can be achieved with a little distraction and a large shoulder bag," said Mr Nicholson.
"This is a warning to our fellow auctioneers - 'be observant, be vigilant', because these people are as bold and cunning as they come."
