I would contact the shop and tell them what happened - my conscience would not let me keep something I hadn't paid for. RootieTootie - I suspect you are asking the question here because you have doubts about whether it's right for you to keep this item, and somewhere deep inside, you know it's wrong. Yes, it was their mistake, but is your conscience absolutely clear about benefitting from that mistake to the tune of £100?
Rasputin mentioned that shops have a 'shoplifting budget' and this will come out of that. In a sense, she's right - shops have a margin in their prices that covers 'shrinkage' - which is loss due to shoplifting, or an item getting broken etc - but the important point that rasputin failed to mention is that the margin for shrinkage is paid for by the customers - it is a bit extra on the price of every single item sold. This isn't going to come out of Halfords' profits - it is going to come out of the pocket of every person who shops there.
I am afraid I do judge people who do this - who say, it's a gift from the christmas gods, it's karma because [insert name of big retailer here] doesn't care about their customers, I spend loads of money there, so I deserve it, the assistant wasn't polite enough to me, so I deserve it, it was their mistake, so it's OK. It's not OK.
As others have said, if the mistake had been the other way around, if, for example, the assistant had charged you for the model you wanted, but given you a much cheaper one, you would have been back in there quick-sticks, to demand your rights - to be honest, I think that if someone thinks it's OK to benefit by a store's mistake, they should take the other side of the coin too, and not be able to complain when a store makes a mistake to their detriment. I have no qualms about complaining if I am overcharged, because I know I have owned up in the past when I have been undercharged for something, or when the tesco deliveryman has left me a bag of someone else's shopping.