It will be very difficult to prove negligence against the local authortity, particularly as there were no previous complaints made and therefore they probably were not aware that there was an issue in the first place.
It would really depend on how bad the leaf spillage was, but the argument that would be levied by the other side (and not unreasonably IMO) would be "Did you not see the leaves?" and "Did you not think that given the time of year, the leaves may be wet and slippery if you walked on them?"
Councils usually defend cases rigourously, and I can see all sorts of potential problems with a case of this kind
