This has just been posted on You Tube with no description about when or where it happened. Anybody any ideas?

 I vaguely remember this video doing the rounds last year when it was posted on liveleak. As I recall a tiny hole in a hydraulic line caused a fine mist of fluid to be sprayed out which then ignited. There were no major injuries and the helicopter was badly damaged but eventually returned to service. I'll take a peek around to see if I can find the actual story.

Thanks Batfink it would be interesting to know! 

I believe I first saw it on PPruNe where someone gave this explanation -

This happened in Sept of 1998 aboard the
"Bonhomer Richard", pronounced with a
french dialect. My squadron was HMM-164 and I was
a CH-46 Helo Mech. We were supporting Operation
Urban Warrior off the Monterey coast in
California. This bird was taking media crews back
and forth from land to the carrier all day and at
this moment they were leaving to go back to shore.
I was in the chow hall aboard the Bonhomer
"***" when this happened. CH-46
crewchief Sgt. Beyers received smoke inhalation
while saving these people and was treated at the
scene. For his actions "under fire", no
pun intended, he received a Certificate of
comendation and a "NAM". After
investigating, it was found that a pinhole in one
of the aft transmission lines was
atomizing(spraying) fluid and the spray was being
sucked into the aft generator and this caused the
fire. Transmission fluid is not flammable in large
quantities but when atomized it is very flammable.
The aft portion of this bird was completely
destroyed and had to be removed and replaced.
After 9 months of rebuilding this bird was put
back into the fleet and renumbered. Before the
fire it was #07 and after we got it back it was
#15.
Cpl Petree USMC 97-02