Disposition of property on death has always been a very complex issue
Two peculiar types of wills are relevant to our case, the first one being the mirror will, the other is known in equity as a joint will, both are mutual wills that arises when two individuals decide to put in writing two identical (or similar, depending) wills that would usually be to the benefice of the other for life, and in most cases to pass the benefit to another (usually their children  ), just as Henry and Patricia did when they made wills in identical terms leaving the entire estate to the survivor and, on the death of the survivor, to their son Benjamin.
The first step in discovering whether Henry and Patricia had a mutual or mirror will is to differentiate between the two.
Mirror wills are fairly uncomplicated on the surface, and are quite common amongst couples. A passes everything to B if A dies first and vice versa, the survivor then leaves everything to a beneficiary, usually his/her children  (this applies to mutual wills too). The canon in a mirror wills is that the survivor retains testamentary freedom, this means that the survivor is allowed to change his/her will after the other party dies, which is very useful, especially when there have been second marriages, and consequently the wills are not dependent on each other for their legitimacy.
On the contrary, mutual wills are dependent on each other. They reflect a contract  between the parties to the mutual will, and it is expected for the survivor to be bound by the terms of that mutual agreement unless revoked before a binding event occurs  . Therefore the survivor holds the estate on constructive trust (or as life interest)  with a duty to bestow the estate to the beneficiary/ies as predetermined in the will, which is part with the contract made with the deceased.
Briefly, a mutual will is written in such a way, with provisions incorporated, wherein both testators agree that the survivor's last will is to remain unchanged after the death of the first party. In our case the fact that the will stipulated that it was made “pursuant to an agreement" between Henry and Patricia for the disposal of their property goes a long way to show that it is a joint will due to the contractual nature of the passage above, since there is no requirement for the contract to be in writing it can merely be an oral agreement and having the word “agreement" together with Their solicitor explaining that by that clause “they were agreeing that neither of them, at any time in the future, would seek to make any further changes to their wills without the consent of the other and that if one of them died, no changes at all would be possible" should suffice  to make it a mutual and not a mirror will, since, “there will be a mutual will arrangement where evidence from the couple’s solicitor indicates that their true intention was to bind one another irrevocably.  " 
Now that we established that the nature of the will, in our case a mutual will, we should further expand on the subject, in particular to those points of law that concern our case.
For a mutual will to be effective three conditions must be satisfied:
That mutual wills must be made pursuant to an agreement  ;
That parties to the agreement accept not to revoke it  ;
And a binding event occurs  .
We have satisfied both first and second point of those conditions at the time the wills where singed in front of their solicitor. At this point we should see what a binding event is, and when dose it occur?
A binding event can occur in number of ways for our case the most pertinent way will be death of one of the testators, as exemplified in the case of Re Dale  , which means that Henry would only be able to revoke the mutual will before the death of his wife Patricia and that; from the moment they made their wills to the moment Patricia died he provided enough consideration for it to be a binding contract and hence a mutual will.
At the death of Patricia, Henry receives the entire estate into his sole possession yet he is seen as holding a “floating trust  " that will “Crystallise" with his own death to a constructive trust towards Benjamin the beneficiary.
Things are quite simple until now however they get a bit more complicated when Henry receives Greenacre and remarries to Louise.
The effect of marriage on a mutual will is of relevant importance since the Wills Act 1837 s.18 provides that a will is cancelled by ensuing matrimony unless it was made in the contemplation of that marriage. If Henry remarried before Patricia died then the will would have been automatically revoked and there would be no possibility for an action  . However in our case Henry remarried after Patricia’s death and therefore the trust has arisen and the testator cannot revoke it by marriage since the courts have held that the revocation of a mutual will does not change the earlier created trust of the mutual wills agreement and therefore remarrying does not free the estate from that trust and as a result the trusts taking place from the mutual wills agreement are not affected by Henrry’s remarriage and Benjamin, the beneficiary of the mutual wills is still entitled to the benefit under that trusts, as commonly applied in court  . 
In Dufour v Pereira  , Lord Camden further commented that:
“A mutual will is a revocable act. It may be revoked by joint consent clearly. By one only, if he gives notice, I can admit. But to affirm that the survivor (who has deluded his partner into this will upon the faith and persuasion that he would perform his part) may legally recall his contract, either secretly during the joint lives, or after at his pleasure, I cannot allow."
Nevertheless if there is other property which is part of the mutual will but is not dealt with as an element of the property under the trust then the survivor will have shown that the will has been revoked to the extent that concerns that other property and can now be placed in a new will or disposed of as Henry thinks best, just like in the case of Re Green  . In short the agreement not to revoke may apply only to part of the estate, in which case the agreement applies only to that part.
This could be bad news for Benjamin since it would mean that the latest addition to Henry’s estate, in occurrence Greenacre, might not be part of Henry’s mutual will with Patricia and therefore Henry would be free to dispose of it as he sees fit, including therefore leaving everything to his new wife, Louise. Yet, Benjamin would remain the beneficiary of the mutual will between Patricia and Harry for their estate as stated in their mutual will as reaffirmed in case law  .
A donatio mortis causa (DMC) is a gift made during the life of the donor which is conditional to the donor’s “contemplation of death", and takes effect when the later dies. This type of death bed gift has been described, by Buckley J in Re Beaumont  , “as a gift of an amphibious nature, being neither entirely inter vivos nor testamentary. It is an act inter vivos by which the donee is to have the absolute title to the subject matter of the gift not at once but if the donor dies".
According to Lord Russell CJ in Cain v Moon   2 QB 283 what we need in order to establish for a valid donatio mortis causa is that:
a) the gift must have been made in contemplation of death;
b) the gift must have been conditional on death; and
c) the donor must have parted with dominion over the subject matter of the gift.
A gift made in contemplation with death means that the donor believes he is going to die in the “near future"  due to some illness, or other perilous factor. In our case it seems that although Henry contemplated death at the time of his gift to Louise, he did not die of the thought cause. This, as testified by Lord Tomlin’s in his opinion on the Wilkes v Allington  , is of little importance since: “If a man, in contemplation of death…in fact dies from some cause other than the disorder which was present to his mind when he made the gift, I have difficulty in seeing why the gift is not operative." 
A gift made must be conditional on death would mean that there is a clear intention to part with the whatever is given only if the donor dies, subsequent recovery of the donor will be held as if the gift has not taken effect, as in the case of Staniland v. Willott  .
Nevertheless, the condition of death does not need to be stated explicitly, for if the gift is parted with at the time of the donor’s last illness and in contemplation of death the court will be inclined, following the case of Gardner v. Parker  , to infer that the donor intended that the gift should be held only if death occurs. Furthermore, even if it seems that that the gift was unconditional, the presumption can be rebutted by the court  .
In Sen v. Headley  2 WLR 1308, Nourse LJ states that there must be a delivery of the subject matter of the gift, or the essential indicia of title thereto, which amounts to a parting with dominion and not mere physical possession over the subject matter of the gift.