LONDON - Looted Nazi gold deposited in Swiss banks was worth an estimated £1 billion less than originally thought, the Foreign Office admitted yesterday. Evidence revealed by Britain last September disclosed that the chief Swiss negotiator at the end of the second World War let slip that the banks held $500 million of Nazi gold - £4 billion at today's prices. But officials yesterday explained that due to a "transcription error" the amount should have read 500 million Swiss francs - 25 per cent less.