National Australia Bank Ltd (NAB) faces an exodus of UK customers outraged by the demands of its British units to make up the difference on mortgage payments that they have incorrectly calculated. NAB could not comment on media reports that Clydesdale Bank, and its sister Yorkshire Bank, had offered compensation of up to STG500 ($A855) to customers undercharged interest on the mortgages due to a computer glitch. But refunds were in store for the 18,000 customers affected as the bank moved to recoup its interest income, the bank said on Friday. "I understand that affected Clydesdale Bank customers whose regular payments are to increase will be eligible for a refund of the interest they've paid on the inaccurate higher mortgage balance caused by the underpayments," NAB spokesman George Wright told AAP.