You think this smells, and maybe it does. But if it is genuine (& how did C's widow happen to find out about it after all this time? - you really need to ask her) then you need to know (as magicmerlin says in his/her very useful post) whether D left a will.
If he did not, then what should happen is that DWP are informed of the fraud by D so that they can reclaim from the account all the overpayment they made to him. The rest of the money (if any) should be shared on the basis of intestacy rules - probably between A's widow (if he had one), C's widow and you.
However, where is the proof that the money in the account came from D? What information will the bank want in order to release the money? How did C's widow find out about it? How much money is involved? Was D committing benefit fraud to receive other benefits as well as Income Support? If he was, and it went on for some time, the money due to DWP etc. could add up to many �000s.