Where -

(a) a person entitled in remainder under a settlement to an estate tail in any land has made an assurance thereof which fails to bar the issue in tail or the estates taking effect on the determination of the estate tail or fails to bar the said estates only, and

(b) any person (other than some person entitled to possession by virtue of the settlement) is in possession of the land for a period of twelve years from the commencement of the time at which the assurance, if it had then been executed by the person entitled to the estate tail, would have operated, without the consent of any other person, to bar the issue in tail and the said estates,

then, at the expiration of that period, the assurance shall operate, and be deemed always to have operated, to bar the issue in tail and the said estates.