Who is to blame?

I tripped over a bin bag which had a large cardboard tube sticking out of it ( the tube being what I actually tripped on) I fractured my ankle and was in plaster for 6 weeks, The insurance company for the school that I worked at deny liability but in a statement a teacher that was in the kitchen with me at the time claims she had seen the tube sticking out of the bag mintues before I came into the kitchen, I was carrying a tray of jugs at the time so I did not see the bag. A group of school mothers had left the bag there next to the bin why they didnt put it into the bin I dont know as it was empty. Is it my fault ?