Mark was away at college for a lot of the time, so how can we know that he was the only person who had access to the family home?
John, how do you explain the fact that there were three layers of concrete concealing Sami's body. Mark admits to having laid the top layer, and this, allegedly, was done in an amateur fashion, whereas the other layers seemed to have been professionally done. Are you suggesting that Mark got someone else to bury his father's body?
Believe me, he would have known as would the neighbours.
As far as the three layers are concerned it isn't a mystery. When someone is intent upon disposing of a body in a back garden of a house located in a housing estate it isn't the best idea to draw attention to it because if it later becomes public knowledge that the elderly resident of said property has mysteriously gone missing...well, you know the rest.
The person who dug the shallow grave and then filled it over with mortar had to do so probably in the hours of darkness while neighbours were asleep. He would only be able to bring small quantities of sand and cement to the house in order not to attract unwanted attention. Thus we have multiple layers of what can only be described as a chaotic mess of mortar. I have no doubt that several trips were made to the house secretly and without anyone noticing. In the end this person, whom we now know to be the victim's son, Mark Alexander, decided he would order a small load of premix concrete and apply that over his own handiwork. The pretext for this effort being that underpinning of the garage wall was required.
As previously discussed, underpinning is not something an amateur would tackle, it usually requires a surveyors report. Thereafter the wall to be underpinned is excavated under and jacked up before concrete is injected underneath and allowed to cure properly before the jacks or props are removed. None of this took place at the Alexander's residence. I am quite sure however that had this work really been needed that the owner of the property, Samuel Alexander, would have been in attendance to oversee it.
As I stated earlier, none of what Mark Alexander claims stands up to scrutiny, I am yet to be persuaded.