They do have to honour the marked price, the way someone like argos gets away with it is very very simple.
Once the mistake is noted, be it by having to sell one at the mistaken price, they will then simple have the item withdue forsale by claiming its out of stock untill the price mistake is rectifed or offer period finishes.
Or like the japanese stock broker that sold stock at wrong price due to typeo, cant remember figues off hand, but it was a very large amount of money.
But because it was so clearly a mistake the sale was voided. It was also beleved the poor trader would kill him self (the japanish harey carrie thing i cant spell) because its traditon in japan for bringing shame on himself.
Have to try digging out the paper.