My son worked out that it was Pierce Harris (who raped Rhonda) He reckoned that it wouldn't be one of the major characters as they would then have to leave the soap.
What confuses me is that at the end of two other episodes this week it looked like Graham was hit by a car and fell off a bridge, so maybe Pierce is not the killer. The actor who plays him has just been on This Morning, but would not confirm if he is, or say how long he was being brought back for. But he did say all would be revealed tonight...