Guys I just want to chip in here and say that lots of people get good scores without buying expensive prep courses or books. Don't panic if you haven't got a grand lying around for a course.
YouTube is your best friend. It will teach you everything. Just make sure you keep on applying knowledge to questions. Any questions are helpful, and gamsat-styled ones are even better. I liked Khan Academy and Professor Dave Explains the best, but find channels you like.
I have an English degree and no experience of post-GCSE science and I got 64, 72, 50, giving a total score of 59 / 62 depending on how you calculate it. It's not the best score, but frankly it's good enough. I prepared for it around my full job, and my part time job, and fitting in enough work experience.
I had moments during my prep where I thought I was never going to be good enough in section 3 and wanted to give up, but you CAN do it and you just have to keep going.