During my stage 1 they specifically said there is no negative marking, but this may have changed.
Also remember that by the end of the day we actually don't know what they are looking for, it may be speed, it may be accuracy, it may be a combination. It may be not having more than 10 faults (random number) regardless of questions asked, and so on.
The only thing that can be done is your very best, exactly what NATS are looking for we will never know, just make sure you do your best because that really is the only thing you can do.