Well you have three days still. Realistically you can't do anything unless they overstay. They may have changed their mind, not have the funds available etc, but as soon as 10am on Monday comes or whatever your checkout time is, they no longer have a right to occupy so I would have your housekeeper there on the dot. She can enter even if they haven't left and assess the situation. Does she have any burly friends?! If not then can you try contacting them at that point, even if it was via your housekeepers phone, assuming she was willing to make entry should they still be there.
Try not to worry, they may be pushing the builder to get back in, using the cost of staying with you as a negotiating tool or asking the builders to pay. All is not lost yet and the Police will be your last resort but you do have that option. Can you turn water or electric off from outside?