Hey guys, nice one Kev on the info here, I can report 100% success with a marble inserted in AIS pipe.
No more backfiring now at all, a little disappointed I can't frighten old ladies now.
On the other hand it was embarrasing that post 11pm people would open their curtains to see who'd been shot.
I did choose a marble that was ever so slightly oversize of the pipe and eased it in with a tiny bit of silicone lubricunt grease.

Oh I forgot to mention, I have a 2011 XT660X. And have just fitted Akrapovic after market cans without baffles in.
Just out of interest, is it my imagination, or could these cans be slightly quieter with the ais pipe now blocked? And I'm not talking with reference to the backfiring.