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Contributory ESA is not affected by any income or assets you might have - The only exception being income from a pension! The lump sum will not be a problem. The first £85 per week of the pension will be disregarded, then 50p ESA will be deducted for each £1 pension. Even if your pension falls below this level, you should still notify DWP once it is being paid. Keep a copy of the letter. That will save a lot of potential problems later on!