If you progress to the assessment centre, you'll be emailed the correct and best(ish) resources to practice with. I wouldn't worry until you have this!
You will only have one attempt per assessment at the one centre, and in total you'll have two lives (two attempts ever) to get through the assessment centres. If you fail an assessment, you'll need to wait at least six months before you can apply again.
There is a Group Bourdon tool on this forum you can download for practising the concentration test. Hope this all helps!