VW polo emissions

Hi my partners 04 VW Polo 1.2 petrol failed the nct on emissions. Brought the car to a garage. They had a look it and said the there was a crack/leak in the catalytic converter. They welded it up and gave us us a can of cf5 fuel cleaner to chuck in the petrol prior to retest. The also did a emissions test on it in the garage. Limit is 0.20 and results in emissions test was 0.17
Anyway brought the car back for nct retest and it failed miserably again. Brought the car back to the Garage again and they said that the lambda sensor will need to be replaced, so they replaced that and since the car has not being serviced in a couple of years they also replaced spark plugs. They retested the emissions in the garage and they said the car is passing now no bother.

I had a look at the reading now and they are CO 0.15. The reading are only 0.02 down from previos reading. Nct now not till next Thursday but I'm a bit worried after all this will it even pass again.

Does this process that the garage have undertook sound right ?