I have realised that I'm incredibly interested in international relations, politics and human rights. It is just a thought but I could consider it as a path that I could take at this critical point in life. I currently study Microbiology, I have loved my course and science is still fascinating to me and I wouldn't want to waste all that I have learnt. I doubt I will act upon this interest, and it will remain just that an interest. But would it even be possible to get onto a masters course and thrive without having a prior degree in a related subject? And does anyone have any warnings of following through with a major path change like this?

If you doubt you will continue to follow your interest in science, then you have answered your question. Just make sure what ever new direction you take it is out of interest primarily, not just monetary considerations. You will always find a way to make money from the thing you love if you are driven.

As for prior training I went from BA Theological Studies > M.Sc. International Trade Policy > PhD Environment & Politics. So you just need to have the hunger for the subject area and you will put out excellent work. It does not take long to "catch up" to a field if you are serious.

It is possible to shift and I'd like to suggest one way in that scientists have taken in the past. That is to go in through security / war / peace studies: these are fields where scientific knowledge can be very useful e.g. your background would possibly lead to an interest in biological weapons. E.g. something like this http://www.kcl.ac.uk/sspp/departments/warstudies/study/graduate/ss.aspx
I would though give one caveat: make sure the academic discipline of IR / Politics is what you think it is. It can come as a bit of a shock. I'd suggest getting hold of a copy of the classic first year undergrad 'intro to IR' textbook Baylis, Smith and Owens 'The Globalisation of World Politics' and see what you think.

Thanks for the advice :D