Fuel smell in car

I’ve noticed for over a month now that I get a strong smell of fuel inside the car when driving slow or still and i hhave to open the windows. My exhaust also gives a popping sound and it idles a little rough. A friend checked my spark plugs and two were wet, but otherwise fine.
My mot just failed due to emmissions, high lambda reading, other two were normal. I have put a cataclean through it as advised and patched a little hole in the exhaust and ‘thrashed the life out of it’ and it reduced the lambda reading but not by enough.

I have recently had a full service, (oct) and a new coil pack. A diagnostic also showed a faulty coolant temperature sensor, so that has also been changed and it’s also had new headgasket around 18 months ago. It’s really frustrating because it drives so well and everything passed the mot but that lambda reading. And obviously the fuel smell is a concern. I must add that my fuel consumption is fine too, I get the right amount of mpg. Do all this things indicate what is wrong? In the garage I went to they were scratching their heads, as they thought it could be the coils but as its been done they didn’t give me any other indication what it could be, and they didn’t seem concerned when I told them about the smell of fuel I get. They were very busy and just sent me on my way with my failed mot cert. I feel quite vulnerable as I’m so clueless they could say anything needs fixing and I’d believe them as this has happened to me before, I’d like to go back with a few possible solutions in mind so I’m as knowledgable as possible before I hand over any money for repairs.

Thank you in advance!