I spent ages trying to find this out Tom, even offered £5 to anyone on here who could find me one...no luck.
I spoke to a shop today about having it cut open, sorted and re-welded looking about £150. My car is a 1.8 TFSI though, at the moment it is pretty much silent so I wanted to subetly change it.
I have done the res already and it just made my car gurgly and not really in a good, refined way, I have also done backbox delete with straight pipe both with and without the res in, without it sounds like an OTT racecar/saxo, with it sounded good but droned to the point of annoyance and headaches.
I have decided to try fitting a mk5 gti backbox (they are the same as mine but with less baffles inside I believe).
I also did a kind of silly experiment of driving hard and spraying cold water inside the backbox (well documented on BMW forums, people do it to brand new M3's!!!!) to try and break down the fiberglass insulation, which has not worked and leads me to believe there is no insulation, just baffles.