Someone asked for a quick howto, so I thought it better to do it as a separate post. There's no photos or anything because I didn't really plan it.
This is just the method I used with success, sure there are other techniques
Warning: lot's of nasty stuff in the catalyst material so do this outside and wear a proper mask (and eye protection).
Tools required:
Workmate or similar
Electric drill
Drill extension
Largeish spade bit (25mm?)
Abrasive flap wheel
Hammer
Large old screwdriver/blunt chisel or similar
Hot air gun
1) Clamp the pipe in the workmate so it's secure and the precat is horizontal, to prevent too much stuff getting into the main pipe.
2) Using the drill, extension and spade bit make several holes in the centre of the catalyst material until you have one large hole, leaving just a couple of centimetres of catalyst material round the inside of the cat housing. Hold tight and take it easy at first. Regularly unclamp the pipe and turn upside down to remove material.
3) Using large old screwdriver and hammer, separate the remaining material from the inside of the cat housing. It's bonded in well so quite fiddly. Again, tip upside down and shake regularly to get all the shards etc out. Once you're done you should have a uniform but rough-ish surface around the inside of the cat housing where the cat material was bonded.
4) Using a reasonably fine flap wheel and the drill+extension, clean round the inside of the cat housing until it's nice and smooth and there are no bits which could come loose or burn up.
5) Now the cleanup. Very important. We need to make sure there are no bits left inside the pipe. Turn the pipe upside down and make sure you've got any loose material out (you'll be able to hear any thing rattling around inside. Get a hosepipe and thoroughly flush the downpipe i.e send the water down through the main cat to wash any bits and dust that may have got stuck, give it a good wash around. Shake out excess water and leave for a couple of minutes to drain out. You don't want water sitting in the pipe for long, so grab the hot air gun and *gradually* warm up the pipe (still in vertical position) until all the water has evaporated out and the inside is dry. You could instead, use some sort of solvent to wash it out (but definitely don't use the hot air gun then!).
great link from Brooner too, how have I missed this 
