Examples of bargain blindness include the lure of cut-price petrol, which may cost motorists more by the time they've driven the extra five miles to get it, and "deals" on return train tickets, when two singles would be cheaper.
Similarly, tempting gym membership subscriptions can work out to be much more costly than pay-as-you-go schemes for those who only visit once a week or less.
Even freebies can mean you rack up unanticipated costs. Free furniture, for example, can cost more to transport to your home than you'd spend buying it new, and free pianos can cost a fortune to get tuned up.
As for supersized items or bulk deals, it's always worth checking expiry dates, since this is often a ploy used by stores to get rid of stock approaching its 'best before' date.
British consumers are cannier than ever but many suffer from bargain blindness.