It is in PlusNets rights to keep hold of the line until they understand where the problem has happened. There is no way to pressure them to release the line quicker - well not officially anyway - or any timescale for doing so. Lady I spoke to seemed unconcerned about Plusnets details being given out by Origin - to be honest I would feel even more helpless if they hadn't/couldn't or wouldn't...
Actually that's not the case. While one may argue that PlusNet acted in good faith, they did so without valid legal authorization to do so. Only the legal owner of the line (aka yourself) is able to do that. The fact that Origin failed to notify you is irrelevant to the fact that PlusNet did not have legal authorization to take over your line.
Happened to my brother just before Christmas, though TalkTalk did notify him. However PlusNet then just resubmitted the order for a second time. However the customer trying to take over his line in this case was known, as someone had moved into another property in his courtyard and was using the wrong address (I figure you have to be pretty dumb not to know the address you have just spend half a million on).
Anyway the mistake PlusNet made was my brother is a salaried Judge (as opposed to a fee paid aka zero hours one). They where made plainly aware that if they attempted to take over his line again they would find themselves in court.
The Ofcom procedures are an attempt to keep things out of court because that is simpler for all around. However by persisting in refusing to accept their mistake and release the line back to it's rightful owner PlusNet are committing fraud (they now know the information is wrong so have no defence about not known the information was wrong), and you have suffered a loss (fraud does not require someone to gain just someone to loose out).
I would demand of PlusNet the release of the line, and compensation, with a temporary service in the meantime. If they won't then call the police and tell them you believe someone is doing it maliciously. I would be tempted to issue a letter before action as well.