It happens surprisingly regularly, but it doesn't get reported by the BBC. I was driving through Portland OR ~4 weeks ago and noticed an RV pull of the side of I5 southbound with the engine smoking. After a few minutes, flames were leaping from the engine compartment. I didn't see any indication that there had been an accident (no body damage, etc), so I assume there was no impact.
Its not surprising really, ICEs are powered and lubricated by highly flammable liquids. They are certainly very reliable these days, but fire is still a failure mode.