Bring back the abacus, that's what we say...Thousands of Halifax customers have been charged the wrong interest on their mortgages due to a computer glitch. Britain's biggest mortgage lender admitted that around 28,000 people had been charged too little interest because of a 'processing error'. The average shortfall is around £175. A further 12,000 have been over-charged by an average of £50. Halifax is writing-off the sums in interest that have been underpaid, and will reimburse in full all those who have paid too much.