I have to admit I struggled with GCSE chemistry, and never enjoyed it as much as the other sciences at secondary school. However, at A level I found the teaching quality was much better, we did far more experiments, and I have done much better than expected in it, although still not quite as good as the other sciences. Perhaps you should read a few books around the subjects you are interested in and have a look at their specification (but don't be completely put off one subject based on one small area of the course you find less interesting - I certainly felt that way about the 'developing fuels' module in AS chemistry). There may also be some short weekend courses or summer schools you may be interested in, although it may be a bit late now...
Remember the 90%+ is UMS, not raw marks - it was possible to get 100% UMS in one of my physics exams last year with only 53/60 - one of my friends skipped a whole question and still got 'full marks' UMS! If the exam is particularly difficult, the UMS points will try to even this out by basically ranking everyone in order, then the top 1% get 100% UMS, the next 1% get 99% UMS etc.
GCSE results are generally a good indicator of how well you can do at college, but I know students who achieved nearly straight A*s at GCSE, then failed to put enough work in at A level and got much lower grades then they expected - one is having to retake the whole year (although he was ill for some of it I think...). As long as you work hard and make sure you thoroughly understand everything, it is totally possible to get into Cambridge
ps. I would delay phoning Cambridge until the end of the month - they are currently very busy with pooling interviews/ giving out the last few offers!