Story is that the M5 lost a load of oil/fluid, some of the cars following went straight off the road. Had a look on Northloop and the people involved actually praise the driver of the M5 - apparently he pulled off straight away and went running back down the track to wave/slow down people whilst his passenger phoned for emergency services - stopped a lot more (but not everyone) from ending up in the barrier.
edit/ too slow this evening, posts are flying in
(and i skim read most of the info on Northloop, it was 26 pages, so excuse me if i've stated some random hearsay!)