Don't worry, lots of successful applicants to Cambridge have a past history of underperformance. Generally speaking, Admissions Tutors are only interested in your most recent course of study - in your case, A-levels. That said, it is true that we can only make a judgement based on the evidence in front of us. If you're taking A-levels over two years, there should be no issue, since you'll have AS marks by the time you apply. If you're taking A-levels over one year, you need to make sure you have strong predictions from your academic referee, and it might be an idea to delay your application until after Christmas, if you can, since this will give an A-level Tutor more time to get to know you (four Colleges - Hughes Hall, Lucy Cavendish, St Edmund's and Wolfson - accept applications up until March 1, in most subjects). You might also want to consider applying to a College that asks for advanced written work, since this gives you additional scope for demonstrating what you can do.