Unfair bank charges and missold PPI are two good examples.
You can recover this money back under the law of restitution.
You have 6 years from the date on which you could reasonably have known of the mistake.
You can claim for restitutionary damages. this means that you can require the bank/insurer etc to hand over to you all of the profits which they have made from your money while they have had it. This could add up to a lot of money.
If a court awards restitutionary damages then the defendant is obliged to reveal their accounts in order to disclose the extent of their unjust enrichment. They are then obliged to disgorge these unjust profits to the claimant.
Follow this link for more info