Under the 1977 Unfair Terms (contracts) Act, penalties for breach of contract are not enforceable
The 1999 Consumer Credit Act (Unfair Terms in Consumer Contracts) builds upon and strengthens the 1977 act
The office of fair trade (OFT) ruled on 5th April 2006 that any bank charge over £12 is automatically deemed to be unfair
And anythng that is deemed in law to be unfair or illegal is not enforceable thus you can force your bank to refund their unfair bank charges.