I believe the "A load of" is separate from the basic saying,
which is "Who Shot John".

"Okay," you may ask, "What's with this 'Who Shot John?' then?"

If that's it, then it's a reference to the situation where
there are competing claims about who gets blamed, like,
"Dick hit me first." answered by Dick with, "No, Ralph hit ME first."
And in either case, it's hard for an authority figure to sort out,
because of the competing and otherwise unsubstantiated claims.

So "It's a load of who shot John" is in effect saying, "We're
being fed a bunch of finger-pointing, with only personal claims either way."

Its origin? Damfino.

Dave Oesterreich