For an essay, I have to analyse an accident, in terms of a theory I learnt in a lecture.
Do I need to reference the lecture I learnt this theory in?
Becuase it's not really my knowledge, but someone else's?
Thanks 
From what I've experienced thus far in essay-writing, the specific examples presented in lectures as theory are almost always derived from a particular, reference-able source. You can usually find this source (in the form of a journal article, etc if applicable) on the web, but in the event that it's not been digitally archived, you might have to head into the uni library to get a hard copy of the source.
tl;dr just search for the source material the specific example you're using is derived from; it's almost certainly experimental in origin so there should be a paper you can reference.
(Nothing better than having to dig up the 1902 volume of the British Medical Journal for a single reference lol, I have never seen so much dust)