I'd just like to share our experience with you.
Our daughter last year sat both, the bucks 11+ and the Watford consortium test. We helped her prepare at home just by going over NEFR material, which was good enough for her to get acquainted with the format of the questions and get an excellent result in the Bucks 11+ (133).
As we already knew this result when she sat the Consortium test, we were quite confident she'd perform well. However, at the end of the exam our confidence was not matched by hers. The problem was that the format of the VR exam was different to what she had practiced, which got her heart racing (consequence: loss of concentration). This was aggravated by the fact that midway through the exam a girl sitting next to her asked to be allowed to go to the toilet. Our daughter interpreted this as a sign that this other girl had finished the test, which, when checking her own progress left her in little doubt that she was doing extremely badly. (consequence; loss of concentration turned into panic). On asking our daughter why would this be so? She answered that she had made sure to go to the toilet before the test and that even if she'd felt that urgency during the exam, she wouldn't have gone until she'd finished..
We learned later that some kids sit the exams "for the sake of it", or, probably because their parents wrongly assume that school instruction is good enough to see them through. Which is mischievously reinforced in the Consortium literature.
We all know this to be wrong and that's why we engage in some preparation, which per-se unfortunately has the detrimental effect of placing too much of a burden on our kids to perform well. On the other hand if no preparation goes along, the tests are not taken "all that" seriously, and unintended disruption may follow.
Lessons to be learned, at least in what it concerns to the Consortium test is to make sure your children do not make anything out of what goes on during the exam. Just to take things at face value. Secondly advise them that the formats used to prepare with are only approximations and that they're likely to find question set differently, but that that's not a reason to panic. Just that they'll need to pay more attention of what is asked.