This is certainly an unusual and fascinating case!
I have no idea whether or not Mark killed his father (either deliberately, or accidentally during an argument), whether he paid someone to do it, or whether he is completely innocent. I can, however, see why a jury would have found him guilty.
I don't think they should have done, because the case could not have been proved beyond reasonable doubt----we do not even know when or how Samuel died. He might even have died of natural causes, and the death covered up, either by Mark, or by somebody else. After all, he was a sick man. Also, Samuel led such a strange life, used a number of aliases, and knew people whose identities we don't know, that we cannot really know who might have had a reason to kill him, or why.
The jury would have been persuaded by the fact that Mark did have a strong motive to kill his father: going by all the reports I have read, his father treated him very badly throughout his life, although the two of them seem also to have been fond of each other.
In addition, I suspect the jurors would have been appalled by the fact that Samuel was buried in the grounds of his home, just as many of us were when Trevor Jordache was buried under the patio in "Eastenders" all those years ago!!! Then, of course, there was the dreadful Cromwell Street case with a number of young women buried under the patio by Fred West. It's vile, but it happens, and I surmise the jurors would have thought that such a deed could only be done by somebody who lived in the house (albeit intermittently).
I have to admit to being a bit suspicious regarding the neighbours, who, going by some reports, disliked Samuel, but who were only to ready to report his disappearance--------to a retired policeman who lived in the village. Why were they suddenly so concerned about their difficult, rude neighbour? How did they feel about Mark? Did they like him, or were they angry about him leaving his father to go to university (and leaving them to keep an eye on his father)?
Who knows?