I would recommend looking over your chemistry stuff, but I would also recommend finding some summer reading to help prepare you. This isn't always easy (my uni are VERY reluctant to let students know the reading list before classes start) but it can be done. I'm already at uni, so I don't do it this way anymore, but before I started uni I would look up the course description for each of my modules, see if the reading list was on blackwells yet (for the current or past version of the module) and then do some reading that way. It really helped me get a grasp of my modules whilst my friends were struggling. I think that will stand you in better stead than just looking over A level stuff.