Hello,
Dont worry about it, I went in the first time not knowing much. I used alot of practise material from amazon, look for books on the group bourdon test. Barron's Mechanical Aptitude is good, I have a few books from how2become.co.uk Its a small price to pay to get there. Just practise practise practise until you are sick of it, if you get to a point that you feel you have done enough then practise some more. I thought you could only apply twice, but spoke to a DTM's (driver training managers) today who said there is no limit to applying. =) Good luck! and let me know how you get on!
Alex