LONDON — The banking group TSB says its chief executive, Paul Pester, has quit after a string of huge IT failures. The group said Tuesday that non-executive chairman Richard Meddings would take over while a search for a new CEO begins. Pester’s decision follows another IT failure Monday that left some customers unable to access their accounts. Earlier this year, roughly 1.9 million account holders were unable to access their digital or mobile accounts due to a widespread IT breakdown. Meddings said Pester will leave with 1.2 million pounds ($1.54 million) in pay and benefits for a 12-month notice period plus a bonus.