The 'Cyber Monday' computer meltdown that affected RBS and NatWest customers as they tried to bag bargains in the run-up to Christmas was just the latest in a string of IT glitches that have plagued the big UK banks in recent years.

But is there a greater problem than the inconvenience caused for shoppers? Melanie Abbott talks to those who have worked on the huge, ageing computer systems at the heart of the UK banking system and finds out that banks like RBS face a massive dilemma - spend billions replacing their 'mainframes' or risk bigger, more serious problems in the next few years.

Melanie finds out about the scale and size of the IT systems behind our everyday transactions as she becomes the first journalist allowed access to one of the secret data centres that power the banking payments system at Vocalink. And she hears from Andrew Tyrie, chair of the Treasury Select Committee, about the urgent need to solve the banks' IT problem before they damage the entire financial system.