1999 Alfa Romeo 156 alfa running rich?

1999 Alfa Romeo 156

OK you will have to bear with me here as I am a 25 yo girl that has NO idea about cars at all! It may seem long but I guess I better just write everything I do know about the problem to aid everyone. I came on to possibly get an idea of what the problem could be so that when I take the car to a garage I don't look like a dizzy chick and get ripped off! I have to take the car on a long journey in 4 weeks so any help ASAP is much appreciated.

Here goes. The Alfa was running pretty poorly and needed to go through its MOT, the guy that done it said the emissions were too high for him to check it and let me take it back. My Uncle (who isn't great with cars) tried to help me out to save money and put 8 new spark plugs in it and a new air filter. It went back for its MOT and passed but the guy said the emissions were still fairly high and suggested maybe I find an Alfa mechanic to do a tune up to find out what's wrong with it? I drove it back from my uncle's (1 hour) and it absolutely chewed the petrol! I could smell it also when I was driving it. It didn't do that before the new parts and MOT but it certainly wasn't driving well.
Because I am strapped for cash at the moment I took it to quick fit just to have them check if it was a petrol leak. They said there was no leak but definitely something wrong as its mis-firing? And he mentioned something about could be doing damage to the CAT? When I said the last mechanic suggested a tune up he said you cant do tune ups on them? Basically he said that he couldn't tell what was wrong with it without pulling the car apart and that it would start costing me :(
Just want to know if it has anything to do with the parts my uncle put in? Could they have been fitted wrong? Or could there be something completely different wrong with it?
Its so confusing when 1 mechanic says 1 thing and another contradicts him!
Any suggestions would be great!
HELP!