There's nothing in your post about whether B left a will or whether he also died intestate (and, if so, whether anyone has sought a Grant through Letters of Administration). The amount due to B under the intestacy rules (or under his father's will if Letters Of Administration had to be sought because the executors of that will were unable to act) forms part of his estate. Whoever obtains a Grant in respect of B's estate (whether that Grant be in the form of Probate or Letters of Administration) is entitled to call in the assets of that estate, including B's inheritance from his father.
So seeking a Grant in respect of B's estate would seem to be the first step here.