Your referee can mention extenuating circumstances in your reference + many unis have a form available to submit extenuating circumstances direct to them.
Bristol is quite picky with LNAT as far as I know, in part because Bristol's admissions tutor chairs the LNAT consortium.
I can barely remember, but I think the actual exam was a little harder in terms of MCQ. Timing was definitely more of an issue. Even 2 weeks of solidly practising and going through questions will help, because you don't have to learn new information, only figure out how to process it to arrive at the correct answer.*