Well heres what i wrote, far less comprehensive than Davids, but feel free to extract what usefull information you can!
There is no science interview and general one, they are all mixed up so you can expect science questions in all of them. I also had a bmat style essay sprung on me just before my st peter's interview. They do ask some pretty specific questions, but what i've realisd is that they are aimed at what you should be doing in biology in december, such as nerves, poisons etc... unfortunatly i only did those this week so had to blag it in interview... Just make sure you know all your as stuff really well, and biology and chemistry up to christmas. There gonna test you on stuff you have no idea on, so don't be too thrown by it, they want to see you under pressure. My warm up question for hertford was, "whats the angle between the minute and hour hand on a clock if it's quarter past three" with the other two interviewers racing me to the answer!
5 a's would be good at as level, but don't worry too much if you don't, i got 3 a's and 1 b, as well as average in the bmat and still managed it.
Not as good as the one above, but thats my advice
Tom