When I changed banks, several direct debits went missing.  Most of these were not a big issue, I contacted each person who complained of a missing payment and sorted it out.  In the case of my credit card company, it was more serious.  I  ended up with penalty payments and interest charges to the tone of over £50.

It's taken several phone calls (and telling them firmly that their offer of refunding half the interest was unacceptable), but we've got there in the end.  I now have printed proof in my hand (in the form of a BACS record provided by First Direct), that MBNA were indeed informed of my change of bank 12 days before they billed my old bank and charged me all those penalties.

They're refunding the extra charges in full.

That's saved me writing a letter to the financial ombudsman.