If the wet stat is stuck open, as Nick W says ,the ecu will fuel for cold start .
If you can get live data that helps ,cheap obd2 scanner /torque app ,that naughty scanner
or similar
see what the ecu coolant temp sensor is showing .
vauxhalls over fuel when cold (or the ecu thinks it's cold )
a duff cts coolant temp sensor can read -40 degrees
you will also be able to see stuff like lambdas switching and all that good stuff
cataclean I have used many times , I tend to put cataclean in a week or so BEFORE mot in 1/4 of a tank of fuel, use it till the light comes on , then , fill with fresh petrol.
getting the cat nice and hot BEFORE emissions test helps alot
check exhaust for blows (even small ones) check breathers and air intakes for leaks /blockages