Bloody hell there's a lot of over reaction on this thread!
The salient point that hasn't been mentioned at all is what exactly are the vendors doing? If they are sitting in the house with their feet up, watching the TV and having a glass of wine then of course they are being ridiculously unreasonable. If they are rushing around and trying to get out, then yes, they have obviously been very disorganised and planned badly - and that is annoying and could have been avoided - but there really isn't a lot to do other than wait.
Assuming the later is the case, what exactly do posters think their suggestions of threatening them to "just get out" by a deadline, changing the locks etc will actually achieve? If they are going as fast as they can to get their stuff out, then they are going as fast as they can.
Of course it isn't ideal, but it is what it is and no one else will be able to magically make them move any quicker. New locks won't make their stuff vanish from inside the house in a puff of smoke.
There are always frustrations all round with moving house. We completed a sale last week (top of the chain, but a short chain of three). The money did not make it to us before the banks closed so we couldn't access it until Monday. But our solicitor had had the money, so we'd have been out of line to say the new owners couldn't move in!
Moving house is full of stress, but some of it is self created by how you react to the situation. Of curse you have every right to be annoyed and frustrated, but unfortunately you cannot physically make this happen any faster unless you offer to help them move their stuff out!