Couple of weeks ago at around FL280 near Southampton, IMC, smooth conditions with weather radar on, we were getting a pretty good St.Elmos's Fire display when all of a sudden the biggest lightning discharge right in front of us. Absolutely nothing on radar. Thinking our radar might be u/s I mentioned it to ATC and 2 other nearby aircraft reported same situation.
Fortunately we were "heads down" as the effect was as if a search light was aimed right through the windscreen.