Don't panic you still have time to work on any problem areas. Children can have an off day for a specific mock. My DD did a mock at Congress House and found it slightly more challenging than another one she had done, but I felt the organisation and the feedback was very good. If you go through the areas she got wrong and then do another one there later in the year that may really help.
I do know some kids that were in my son's year who went to the institution you mention and didn't have a great outcome, but that could have been for lots of different reasons. Do you have time over the summer to work with your DD on the areas she is finding hard. There was a good thread with lots of advise from when my DS went through it which may help.viewtopic.php?f=18&t=24928