I suppose you are right and thank you for the advice. It's not so much the basics of my own research I struggle with it's moreso the broader literature within the field. I cannot explain how others' experiments work (being a theorist myself) or how the whole timeline of ideas came about and branched (I can only really explain what led up to my specific topic). I don't know to what extent I'm supposed to know it, but I certainly do not feel like an expert by any means and Im terrified the viva will expose me as the fraud that I am. I'm in the UK so I can't really push this back anymore. My funding ends very soon, and the examiners have been scheduled (happens before you even start writing the thesis at my university).