any 2.2 DTi Chippers aswell?
Have a chinwag with VXL V6 - he's fast becoming the DTi expert here 

Have a word with Pete.
http://www.omegaowners.com/forum/index.php?action=profile;u=2259, he knows far more about this subject than me!
The basics are to decat the exhaust first (only a smog test on a diesel, not emissions) which I got Loo Knee to do on both of ours by basically stripping the cat internals and welding it back together - this way if a visual inspection for confirmation of a cat being present ever reaches the MOT regs, the tester will be able to tick that box! Once you've decatted it you then need to get it mapped by a specialist or look out for a P-Box that plugs in between the fuel pump and wiring loom.
Decatting on it's own improves the sluggish pull away from standstill, improves the MPG and makes it more responsive at motorway speeds, remapping on top of this improves all of that again.
One thing I would get done before any of that is to get the inlet manifold and swirl flap inlet thoroughly cleaned, according to Vectra owners, this can make a massive improvement to the responsiveness of the car. It's the next job on my list.