Have you closed the sale yet? The solicitors should hold the money until the documents are signed and the sale is closed.
At that stage, the vendors should be out of the house. There are a number of issues if they are not out and the sale is closed, among them : a) they could squat, and you'd have a problem getting them out; b) if there is an accident (say someone falls down the stairs), they can sue you as you are now the owner of the property. We were strongly advised by our solicitor not to close until the vendors were out, and we had the keys. In the end, we told him they were out on Friday night, but informally waited until Sunday to get our stuff over (leaving Saturday free for the vendors to move out). It wasn't ideal, but we trusted them.
I don't think you can force them out of the house, as it hasn't been sold to you yet. I also don't think they have to move out at this stage - i.e. they could still back out of the sale. Your solicitor would be the person to ask.