Remember, no matter what your position you will not be allowed on a streetworks or other construction site if you do not have a CSCS card.
If for some reason you should fail the test you will be allowed to take it again. However, you will need to wait 48 hours before re-registering and you will need to pay the registration fee again.
People typically fail because they underestimate the difficulty of the HS&E test. Don't make that mistake. Make sure you purchase available revision materials and study hard in the days leading up to the test.