I think Alain and Silvie are definitely involved. They are the only ones who knew Tony and Ollie were going swimming and the only ones who could get hold of the scarf. The scarf (and the fact he wasn't wearing it when abducted) has got to be a massively significant clue in this (or there was a massive screw up!). But I also think Silvie is trying to get the truth to come out now, hence the facebook picture of the scarf sent to Tony in 2014.
There therefore must be someone else involved who does not want the truth to come out now. Someone who had Ziane killed before he could speak. What does Ziane know they didn't want coming out? Only about what he picked up from the drain- which was either the medal or was swapped by Malik or Mark.
The medal, although the same as the ones Alain had and that Silvie showed Tony, isn't necessarily his. Is this the scriptwriters trying to throw us off scent - Alain can't have been the only one in the AA...
Vincent is definitely significant. He knows something vital although he might not know he does. I don't think he was actually involved or knows what happened to Ollie though. Ian providing his alibi might be as simple as Vincent didn't have one and if he is interrogated too much, Ian's behaviour might come to light. I wonder where Vincent actually was that night. Was he at the pool after all? Maybe with Ian.
The Romanian gang have to be connected somehow. I don't want them to be though as they are boring.
My nice little theory is that Alain and Silvie abducted Ollie for their childless daughter Monique, which is why George cannot see much of her anymore. Series 2 is the agonising decision about whether Ollie stays with his new Mum and Dad or returns to the divorced Emily and Tony and whether the whole Ian killing might come out.