In hindsight, the migration of customers from the British bank TSB in 2018 could have gone a little more smoothly. It reckoned it would take a weekend, and so at 4pm on Friday, April 20, the IT team began to transfer the records and accounts of 5.2 million customers from the system of its old owner Lloyds to that of its new owner, the Spanish bank Sabadell. Customers were warned that a few services, such as online banking, would be offline until Sunday at 6pm, but some clients began to get that sinking feeling when they logged on and saw the wrong balances, or saw the accounts of other customers. The trouble continued on Monday, with hordes of customers complaining on Twitter that they couldn’t get into their accounts. Sabadell then issued a statement, unwisely, saying it had successfully completed the technology migration. Tuesday brought more pain, with about 1.9 million customers locked out of their accounts, to which the response from chief executive Paul Pester was that he was ‘deeply sorry’. Incredibly, one month later, some customers were still not getting into their accounts, and when the smoke cleared, the bank had lost 80 000 customers, £330 million, as well as Paul Pester. A report was commissioned (undertaken by the aptly named law firm Slaughter and May) and revealed a litany of errors and oversights, such as the plan for a ‘big bang’ migration, insufficient planning and testing, and poor internal auditing and communication.

As it happens, Absa has just completed a similar project (codenamed ‘Valpre’), and, as CTO Andy Baker puts it, it was decided, ‘we’re not doing it like that; we’re not going to go for a big bang’.