im fairly sure they have to honour the price they had it up offered at originally,but i aint sure on the legality of it tho
my mrs bought something on line a while ago, a pair of shoes [there was a 100 quid difference] and theyd priced them up wrong she kicked up a right stink about it and long story short, she got them at the original advertised price.
It would be interesting to see what the legal obligation of it is,if you could be arsed looking into it.