Just a word of warning!
See if you can find out if your car has a pre-cat fitted before you go through with the "removal". I downloaded the pre-cat removal procedure off this website, bought the required tools and on the weekend finally had a window of opportunity to do this. I was hoping to find a collapsed pre-cat or at least if it was intact I would remove it.
Don't know if it's much different on the Zafira but I spent 1-2 hours bashing away at the muffler (rubber mount bracket) trying to separate the pipes at the clamp joint. In this time I managed to get them about 4cm apart. It didn't seem to wanna move any further apart than this so I ended up unbolting the rear muffler and taking the whole system down and off the car. It was at this stage I wished I had looked down the engine bay to check the state of the pre-cat once the manifold joint was off. Because standing on the side walk with the whole exhaust system in my hands I realised that there was no pre-cat fitted!!
I next found myself resting the rear axle bend on the neighbour's fence bashing the end of the cat with a mallet to bring the clamp joint back together. I luckily got the whole ting together without breaking anything...
I'm sore and bruised from crawling around under the car (Only had jack stands at the front beams) and after a day of labouring I achieved nothing apart from a few hammer marks on the cat and muffler.. Definitely do your research before "removing" the pre-cat! My Zafira is a 12/2004 production...