Congratulations on receiving your offer to read PBS at Downing. Now that your examinations are over, you may like to start preparing for your arrival in Cambridge with some reading, and thinking about the optional papers that you would like to take in your first year.
As indicated on the PBS website, there is no pre-requisite preparatory reading that needs to be done. However, the website does list some texts
(http://www.pbs.tripos.cam.ac.uk/prospective/faq#section-8) that might provide a good general background to some of the subjects. Additionally, the optional papers provide suggested reading lists - see the individual course information on http://www.pbs.tripos.cam.ac.uk/current/partIApapers. You probably won't have time to do all of this reading! However, you may want to browse some of these books to help you in making your selections of courses.
Another useful approach would be to revise your Maths. First year PBS students will cover maths in the Psychological Methods paper (PBS2), so reminding yourself of your GCSE Maths courses would be helpful. Many students find that maths can take a disproportionate amount of their effort, and a few weeks of revision before October can make the difference between staying on top of the course and struggling to keep up.
It would also be useful preparation for PBS2 to revise your GCSE and A-level Biology notes on genetics. If you have not done A-level Biology, then you may find it useful to read through the sections on genes and classical genetics in the book, 'Essential Cell Biology' by Alberts et al. As stated on the course website, A-level Biology is not required to study PBS, but some reading on this topic will help you to be better prepared for the early PBS2 lectures.
The months between completing your examinations, receiving your grades and, hopefully, coming to Cambridge are going to slip away all too quickly, and I look forward to seeing you soon. In the meantime, please do not hesitate to contact me if you need any further information.