Do Unis vary in how hard it is to 'pass' the interview for medicine?

Yes. There are numbers here: http://www.thestudentroom.co.uk/wiki/Medical_School_Competition_Ratios

Bear in mind that interviews can be substantially different in what they involve too - you've got classical interview (why do you want to do medicine etc), multiple mini-interviews (MMIs), academic interviews (Oxbridge), etc etc.