Hey, its called an 'olympiad' - hence it requires A LOT of practise and prior knowledge: it is designed to be very hard and for only the very best to succeed. Hell, the point of the test is for the RSC to pick out the best 16 candidates in the country so they need to distinguish between them!
If you aren't applying to Oxbridge and your in year 13 and you didn't do great, you have nothing to worry about (even less if you're like me in year 12).
A lot of the people who take this test are the best few in their school and will have had a lot of preparation by their teachers/ at home prior to the competition, so please don't let a poor result in this rather extreme chemistry challenge get you down!