Were there any warning signs for the diesel? If not:
Ask the council / police if a previous accident was recorded where you had yours, if there was the diesel could be a spillage from an earlier collision.
It is the councils duty to clean up or place warning signs for diesel on the road, if they failed to do this and were notified of an earlier crash then the blame is theirs and you could be compensated.
The only reason I mention this is because my sister worked in insurance and had a situation where a claim was put through to the council for their negligence in not cleaning up diesel after an accident.