I spent some time looking at the make-up of the bladder - I referred to Gadget's page but the how-to there shows a different bladder with a slightly different lay-out. Here's a guide to gutting your catalytic converter on your Vulcan Mean Streak.
I looked inside as much as possible using a torch and worked out where the wall holding the honeycomb was fixed - I could also see the weld marks on the exterior. I then marked up where to cut. I used an angle grinder to do the cutting, a dremel cutting wheel lasted about half an inch so although it cut much neater than the angle grinder it wasn't really up to the job.
I didn't want to remove the whole of the "wall" holding the honeycomb as it will still create a little bit of back pressure and I figured I didn't want to have too little back pressure. This might not actually make much difference but hey ho, I only removed the honeycomb.
To start removing it I bashed a hole through it with an old screwdriver, it's thicker than I expected at around 1.5".
I kept chipping away at it, pulling out bits with a pair of long nosed pliers, twisting around rather like opening a tin of sardines. It took a fair while to do it this way. I changed tactics and used my screwdriver to force between the honeycomb and the circular hole it sits in. After getting about half way around I was able to twist the hole thing out of the hole and ended up with this:
I cleaned up all the edges and it's now ready to be re-welded together.
Job done, time for a cup of tea :-)