So. How about this: The Tar Man = Gideon's (supposedly dead) eldest brother?
You know, I've been thinking about it, and I just can't seem to remember where I got this from. It didn't dawn on me until after I had finished the book, so it was probably something at the end. Buh.
According to Gideon, shortly after his father died, his eldest brother also "died suddenly". After this, his mother picked up and moved them to Abinger. Why would she waste money moving somewhere else, when they had a perfectly good house in Somerset, even if it was near her sister? Perhaps to shield her children from knowing the fate of their brother?
Additionally, both Gideon and the Tar Man had large families that were killed by scarlet fever. But I don't know how common that was back then, so who knows.
Plus, Gideon would be 6 at this time and probably old enough to remember his brother and what had occurred. Though perhaps his mother had lied to them about his death? And maybe, despite his memory of his brother, he just never made the connection between him and the Tar Man? Or he has, and he just hasn't said anything about it yet.
EDIT: Wait, no. Actually, as Joshua was 6 1/2 when Gideon was 10, the whole brother dying/moving/mother remarrying/Joshua being born deal would've had to happen over the course of a year and a half. I don't know how much this ruins my theory, but let's just say that Gideon doesn't know what the hell he's talking about. Okay?
Anyway. When the Tar Man tells how he got his scar, he said that he was nearly grown, probably early teens, and that his brother was 2. I'm not sure if Gideon was the youngest by his father, but if so, this is rather interesting. Though, he says that his brother died before his 3rd birthday-- or so he's heard. He's been inquiring about them?
Still, it's interesting that the eldest brother's death was noted. It could have been written that the family moved after the father's death, and that the rest of Gideon's siblings just died of the fever. The parallels between their back stories seem too similar to just be coincidences.
And one more thing, which may or may not have any bearing on my theory: Just what was the Gideon-related information the Tar Man had received from Lord Luxon's new gamekeeper, who had been spoken to of "the matter"? My guess is simply that he had learned that Luxon set Gideon up, which certainly wouldn't have gone down well in the Tar Man camp. But then, do you really think he would have had to consult a gamekeeper for this bit of information? He had to of known about it beforehand. And I seriously doubt that he'd be yelling "Is it true!" over it, seeing as how Luxon's plot was painfully obvious at that point. It had to of been something else.
There's still the matter of the fact that the gamekeeper was from Abinger, and that both the author and the Tar Man felt it necessary to mention something so trivial. But then again, what could this gamekeeper possibly know about Gideon and the Tar Man's past? Even if he knew Gideon or his mother, how would he know that the Tar Man was related to them? Unless Lord Luxon told him, though the reasoning for such a thing is rather dubious: "Ah, I see you're from Abinger? My henchman had family from there." Yeah.
OH, and one, one more thing. When Gideon explains that it was just all the flashbacks that provoked the Tar Man to defend him at his hanging, Sir Richard says, and I quote: "Although I wonder if there is more to the Tar Man's actions than you suspect."
Just what is he implying? What could he possibly know of the matter, unless he's heard both Gideon and the Tar Man's back stories and has drawn the same conclusion I have? Maybe I'm just reading into things too much?
Or maybe he's just implying that the Tar Man's actions were motivated by looooooove? "I'm so glad the Tar Man chose so opportune a moment to discover his fondness for you!" Sir Richard is awesome. XD
YEAH, I'M STILL GOING THERE!