I can't see why to be honest

Incapacity benefit is a non-means tested benefit, it is payable based upon your circumstances, not income. The claim for Jobseekers may have been based upon income and money available to you at the time of the claim.
I don't claim benefit, but for the sake of argument, lets say I did. If I was to have a lottery win three years from now, would the government demand back any benefit money I would have (hypothetically speaking) claimed? No.
Unless your compensation was available to you during the period you claimed benefit, I cannot see why they would want to claim it back from you. As far as I am aware (I MAY be wrong) the DWP can only recoup money if:
1. You were found to be commiting benefit fraud
2. You had a repayable Social Fund loan
3. You are later discovered to have had available funds during the period of your claim for Income based benefits
4. You were awarded a Funeral Payment from the Social Fund and it was later discovered that there were or now are monies available from the deceased's estate to meet the full or part cost of the funeral.
5. You have arrears of child support to pay.
Were you claiming Contribution based jobseekers allowance or income based jobseekers allowance? With jobseekers, the sum you can claim can be affected if you have savings over £6000. If you have savings over £16000, you don't qualify at all. Neither of these sums are anywhere near what you say the compensation is. You do have to tell them about a change in circumstances but this is for the duration of the claim, not after.
Your best bet would be to ask them yourself, otherwise you could have that money sitting around doing nothing. I really don't think they can recoup it.