As rs2ooo custom and a couple of others have said the mot duration is for 12 months i have been informed wrongly, i have just spoken to vosa and they have clarified this.
However the test failure will go onto the vehicle test history.
The only exeptions to the 12 month rule are prohibitions issued by either the police or vosa that req a full test which if the vehicle fails cancels out any time left on the mot.
The guy at vosa did recognise that the uncertainty around the op's original question amongst testers and the genral public is very common because common sence would say if you present a vehicle for a test and it fails the vehicle is not roadworthy from that point. unfortunatly common sence and the secretary of state for transport and their various departments make and approve all the rules regarding the mot scheme, so we have polotics and common sence together and they come up with confussion.........
Anyway sorry to all those i have told they were wrong i am off to eat some humble pie now