Bad luck, sorry to hear that. Quite probably a commercial's diesel oil which often crops up on bends when overfilled tanks with a leaky filler cap slurp some out. It only needs a tiny amount to spread over a wide area, especially if the road is already slightly damp.
It's actually an offence to have any possibility of leakage which drivers or operators can be fined for. You can notify your council who are responsible for washing down the road to get rid of diesel spills and the like, and I've done that successfully in my area.
On one occasion when the council person I spoke to didn't give me much confidence, I chased up the police as well and asked them to put a boot behind them. There was a truck and council crew there within half an hour, and better still, the police had traced the leak to a nearby parked pickup and had collared the driver.
These leaks are no joke, they can be lethal to two wheeler riders and even cause motor vehicle skids.
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