@RichardBuck @henddraig well I'm not sure if I saw all the relevant episodes, and also my memory of what I did see is fading, but ...
I think Jac had been in prison, and had presumably changed his name and moved away from his home area in order to make a new start. I think he'd also become estranged from his family.
For some reason, David took a dislike to him (perhaps seeing him as a rival in Dani's affections) and started investigating his past. He discovered the fact that Jac had been in prison and that Jac wasn't his real name.
(Also, at one point, we saw Siân looking at his picture on a tablet one day, implying that she had discovered his real past also, but that didn't seem to lead anywhere as a storyline. I don't get the impression that he'd escaped from prison and was on the run or anything like that.
That's about it really, apart from the tussle that David and Jac had, more recently, leading to Jac's getting knocked unconscious and going into a coma. David (aided by dad Ron) left the scene, making it look like a mugging. David thought he'd got away with it, but when Jac woke up, he made it clear to David that he knew he'd been responsible, and had stolen his phone and wallet to make it look like a mugging. David had to buy him a new phone. Maybe he gave him some money to compensate for the loss of the wallet. Jac didn't seem to let on to anyone else about this, but it's something he's "got" on David, just as David has "got" something on Jac (his past record).