I have found a U.K. article saying that the Crossfire engine runs very clean and it could be possible that the exhaust gas emissions may be so low that the car would pass the emissions test without the 2 adjacent cats. However the U.K. emissions test requires that the cats must be present and have not been tampered with.

It has been suggested that the two centre line cats are removed, the comb that is causing the rattle is removed, the cats put back and nobody would be any the wiser.

It would be down to the emissions test to see if this is actually the case.

I will keep in digging for more info.

Many thanks