On 17 October it emerged that the transformer fault was due to an oil leak, spotted some weeks before the blackout. The oil had been topped up but the leak had not been cured. National Grid's Director Of Transmission admitted to a "small backlog" of maintenance checks. On 21 October it became clear that the leak had been known of since March. National Grid had started a regular programme of topping-up until the transformer could conveniently be serviced. Evidently this was ultimately not enough to keep up with the leak; "it got bigger" said a spokesman.