My father-in-law asked this question recently, i asked the MOT place i go to, and they said they don't know! It is a very grey area.
As far as i'm aware, like Len says, the old ticket is still valid, however if you are pulled by the police, and you are knowingly driving the car with a dangerous fault, then you could be prosecuted.
This would apply anyway, as the MOT is only actually valid as safe on the day of the test.