The systems breakdown at TSB is not (yet) the worst UK bank computer cock-up of all time: that prize is held by RBS, whose problems in 2012 afflicted customers for a month and attracted a £56 million fine. But the failure of TSB to migrate four million customers’ accounts from systems bequeathed by its former parent Lloyds to a streamlined IT structure, designed by new Spanish owner Banco Sabadell, is surely the fiasco that has been longest foreseen.