Unfortunately, the beneficiaries are liable to refund it. Were any receipts and indemnities given and in what terms?
The pension company can choose to pursue one or all of the beneficiaries - clearly it is fairer if each pay their share.
I imagine a professional executor would have published notices under s27 Trustee Act, which expressly provides that the person to whom the money is owed can trace it to the beneficiaries. Those notices only protect the executor from having to refund it personally. Even if he hasn't published those notices and the pension company went after him, he still has a right of reimbursement from the beneficiaries.