Rattling Exhaust

My exhaust has never been the quietest, but recently it has developed a very loud rasping rattle at revs above 3000. Rather than the sound of a purring engine as you pull away, you have the sound of a rattling tin can - not good.
I went up to GBS last weekend and saw Richard@GBS. He said that occasionay the inner sleive within these new single can exhausts, can move backward and rattle against the rear of the outer casing. He gave me a few stainless rivets and told me to place a rivet about 2cm from the seam and somewhere that can't be seen. This should stop the inner sleeve moving.
Well as you can see above (photo taken from below), I have done this and the awful noise is still there.

Update: It helps if you position the rivet in the correct place. I missunderstood the instructions. The rivet should go about 10mm along the tail-pipe section from where it meets the can. Having done this the rattling stopped - not altogether - but much better. Perhaps another rivet will completely fix it, but I am loathe to keeping drilling holes in an expensive exhaust pipe to resolve a GBS design problem.