J
Welcome to the forum.
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We can't give detailed financial advice on the forum you need to get face to face advice before you make any decision.
What deductions are made will depend on whether your ESA is Income Related of Contribution Based.
For ESA(IR) any pension will be deducted £ for £.
For ESA(CB) there is an £85 per week disregard followed by £1 per week deduction for every £2 in excess.
Any lump sum will be disregarded for ESA(CB) but will be treated as saving for ESA(IR).
Gordon